It may perhaps be worthwhile looking at the continental and US ranges of wagons as some of these may be suitable for use on British layouts, particularly the post war era.
American practice favours the use of auto-couplers, rather like those fitted to N gauge models, which serve to both hold the stock together and also to hold it apart, hence you will need to add buffers to American and many continental models . Buffers are available from, amongst others, Parkside Dundas, but in my case I have used buffers cut from redundant Lima models, the body having been fitted to a Peco chassis kit.
The Lima or Arnold twin-silo wagon requires only a re-paint to produce an acceptable British Railways 'prestwin' wagon (see entry under Lima above).
The more modern types of bogie used on British rolling stock are produced on continental models, although this can prove an expensive way of obtaining these. Modern British bogies such as the Gloucester Ridemaster resemble foreign designs, the Ridemaster type is used on the Lima bogie-tippler wagon for example and similar designs of bogie are available on American models. The French Y25 design has found favour with British wagon builders, these are made by Roco and can be obtained from Taylor Plastic Models or ordered from Germany via Howes of Oxford.
One obvious example of foreign stock on British railways is ferry traffic from the continent, companies such as the German `VTG', and the multinationals Transfesa and `Interfrigo' (refrigerated), lease wagons to shippers, and quite a few foreign railway company wagons have been seen in the UK on this traffic. Roco offer a long bogie sliding walled van in their range of a type regularly seen in Britain on internal traffic as well as on ferry duties. This was originally released in the VTG Ferrywagon livery (reference 2367A) and now I believe in the Transfesa livery. Roco also offer a van (reference 2304S) which is regularly seen in the UK thanks to the ferry services and the Fleishmann sliding wall van reference 8335 is another continental design seen on British lines in the 1980's and 90's. The only problem with the continental models of ferry wagons such as the Roco VTG Ferrywagon and Transfesa vans is that they are to 1:160 scale, and end up being a bit short, which does not show too badly and about 30 thou too low, which is quite noticeable.
The continental and American ranges have few models suitable for use on pre-world war two British layouts, but there are a few which have possibilities. The Roco fish-belly bogie flat wagon (2352S) does resemble a couple of designs used by the British railway companies since the 1920s (often fitted with four bolsters). Fleischmann offer a four wheeled well trolley wagon (8203) which can serve on British layouts. This vehicle can with a little difficulty be made to represent the GWR or LMS chaired wooden sleeper wagons (the GWR wagons were later fitted with sides and used to carry concrete sleepers).
The Minitrix long wheelbase van which forms the basis for GWR carriage trucks and GW/LNER grain/cement hopper vans is also available in their continental range (reference 353400) Roco offer a similar van (reference 2321A) and I believe the Arnold van (reference 4410) is a model of a similar wagon. Longer version of this wagon are available from Roco (reference 2329C) and Minitrix (reference 3239). These could be used to knock up something resembling a long wheelbase GWR/BR DAMO or ASMO CCT and I believe the continental vans were regularly used for ferry traffic into the early 1980's.
Other continental models which might be of interest as ferry or channel tunnel traffic include: Arnold; French STEF van (4564), German open wagon (4206), four wheeler LPG tank in Shell livery (4510), chemical tanker (4523) refrigerated van (4563) and wine tanker (4360).
Roco; VTG four wheelers with large (2320A) and small (2332A) diameter tanks, SNCF van (2329C), open (2332C) and stake car (2308B) and German stake wagon (2308S). They offer a Swiss van (2362E) and also a bogie tanker (2364A), which has a smaller diameter tank than the Lima and Fleischmann alternatives. The Roco bogie flat wagon (2352S) resembles some British 'fish-belly' designs dating back to the 1920s.
Fleischmann; Eva bogie tank wagon (8405), a Belgian open wagon and four-wheel Eva tank (8405).
Ibertren; Transfesa van in blue (382) and white (381) as well as a rather neat two-hatch wine tank wagon (351) and a bogie butane tanker in white Spanish Butano livery (362) which is relatively easy to re-paint for British use. The Ibertren car transporter (452 with cars or 451 without cars) is a bogie vehicle which bears a passing resemblance to the British Procar 80 (The Arnold transporter is too 'flat' for this).
Lima; Interfrigo van (320464) and a continental liveried twin silo wagon (320731, 320732 and 320733), this is the same as was available in their UK range and they also offer their articulated car carrier.
With BR emphasising the privately owned or leased wagons these days there are a wide range of designs entering service, many of which are actually produced by continental companies.
The recently introduced Spanish range by Ibertren, which feature blackened wheels, represent excellent value for money compared with other continental makes, one of their grain hopper wagons would serve well as a TSL `Polybulk' wagon used in the UK for grain traffic and on international ferry trains taking china clay to Switzerland.
I have several odd wagons filling gaps in my RTR fleet of pre war stock. The Bachmann Old Timers series of bogie wagons boasts a pleasing short (about 35 foot over headstocks) chassis with diamond frame bogies and a selection of body types. The `stake car' would serve for a pre-BR wagon although you have to add buffers. I removed the stake holders on mine and replaced the sides with strips of plastic card for use on a 'light railway' layout. It could be used for a Big Four era layout and although it does not resemble a specific prototype, I can always claim it was `absorbed' in 1923.
Unfortunately many Continental and American models do have rather shiny wheelsets, but I have had some success painting these with varnish mixed with some dark colour - I had to polish them up a bit afterwards to get smooth running but the visual improvement was worth the effort. An alternative would be to replace the wheel sets with the Mike Bryant or Parkside Dundas types but I have not yet tried this.
The body will often simply un-clip from the chassis, so do not disregard continental wagon types in second hand windows, the chassis may suit a British prototype, an example of this might be a BR steel wagon fettled from card on the Lima tippler wagon chassis. Bernard Taylor built a very pleasing BR ferry van using a Roco stake wagon chassis, and the Bachmann old-timer chassis would serve for a fairly simple scratch build of an LMS (ex Caledonian Rly) 30 ton iron ore wagon per Roche drawing G/121. The ends could be taken from the Peco 5 plank wagon with sides made up from scribed card (you will have to scribe to plank lines to get the height right, it is taller than 5mm). I am fortunate in having a supply of old Lima chassis to use for buffer beams but 'dill' pins with a strip of paper round them can be used for this. These wagons operated into the 1950's on British Railways.
Those people modelling a Light Railway, such as those run by British Colonel Holman Frederic Stephens, can use the older US or continental coaches. Note that goods wagons and vans tended to be to standard British designs on all standard gauge lines. The Arnold range includes two six-wheeler coaches (reference 3041 and 3043) and two old style bogie coaches (reference 3055 and 3056) which would serve well. The Fleischmann baggage van (reference 8055) or six wheeler coach with guards look-out (reference 8094) would look reasonable tagged on the tail and the Minitrix T3 0-6-0 loco (reference 2047 or 2914) would serve for a light railway loco. This loco uses the same chassis as the Minitrix British outline dock tank engine.
Rather more difficult to obtain but well worth considering are the Japanese ranges from firms such as Kato, Tomix and Green Max. I have a couple of Kato six wheel diesels which served as shunters on a docks attached to a layout. Beware however - Japanese models are to two scales, those representing stock running on standard gauge track are to 1:160, those representing stock running on the narrow gauge lines so common in Japan are scaled at 1:150. As a result my 'little' dock shunters were a bit large compared to a British engine (for example the 04 on the Minitrix chassis), but as docks often had a few big engines this was acceptable for my purposes.