Reference your train set idea - I had a rummage and found some odds and ends you can have to avoid spending money until you are sure this is something you want as a hobby.
What I have is all somewhat elderly (it does all run okay though) and 'European' rather than British.
One option would be modelling the railway in the Duchy of Grand Fenwick (from the books The Mouse that Roared and The Mouse that Went to the Moon by Leonard Wibberley).
Purists will tell you to 'model a prototype' rather than just make something up, but really there are so many compromises in modelling that basing something on a real world example just means you have some kind of structure to work from.
Creating your own structure is equally valid providing it explains everything on your layout and the layout itself conforms to whatever standard practices apply on the notional prototype.
To give yourself complete freedom you could have the Duchy bordering on Ruritania (another fictional country from the book The Prisoner of Zenda) which is connected by the railway, Ruritania might then have a rail link to Germany.
Below is a rough 'back story' for the layout that sets the location and explains the facilities provided, some elements have been included simply because I have the necessary parts in my bits box.
This is based largely on my memories of the 'Mouse' books and rather more vague memories of reading 'The Prisoner of Zenda' as a child. I Googled them to refresh my memory but there is little relating to railways in any of the books, so you have a pretty free hand.
This was written rather hastily and you could probably write something better, but it might be handy to have at an exhibition when someone sidles up with a suspicious look in their eye and demands to know what prototype it is based on.
If you do take it to exhibitions you could decorate the layout with posters advertising Grand Fenwick's industries and attractions, keep a straight face and claim to be the Grand Fenwick International Model Railway Touring Team.
Current Fenwickian railway rolling stock is branded with the New Improved Corporate Logo and goods stock is to be painted in a standard colour, a stock of Soviet era Russian camouflage brown paint having been acquired at an exceptionally good price. Passenger stock will continue to be painted in any colour considered appropriate by the Railway Company, depending on what is available at the bicycle repair shop.
The proposed New Corporate Logo, another product of the Master of Heraldry's degree course, consisted of a rectangle with inward pointing arrows at either end, reflecting in his words 'the essentially internal focus of the system's modality and purpose'.
Unofficially the four outward facing pointed corners also served to underline the often prickly relations Grand Fenwick has with its closest neighbours, France, Switzerland, Germany and Ruritania.
The Master of Heraldry was rather pleased with the new logo and when interviewed in the lounge bar of the Grey Goose Inn he stated that . . .
'This design, devoid of traditional but outmoded features of elucidative textural identification, reflects the current thinking in terms of non linguistic centric trans-national corporate branding, harnessing a dynamic arrow styled motif in which the symbology serves to emphasise the synergy generated between the two ends of the main line.
The original proposal was to have the existing markings removed but the two designs have now been combined to produce the New Improved Corporate Logo, not least because the older markings did not then have to be painted over (there was some doubt about matching the original body colours as the bicycle shop had since changed suppliers).