Thanks for everyones comments, I might be easier to put something here than to respond to everyone.
So the layout will go under my bed for ease of storage. This is useful but naturally gives a height limit of 6''. Regarding having something at the centre, while 38'' is long with the layout not being against the wall means I can access all sides. But more importantly for me the construction of the scenery and model buildings is just as, if not more, important than running locos so I would like the keep that.
In terms of prototypical modelling I intend to scratchbuild bridges based on the one at South Cerney and and the, now demolished, one at Chedworth using brick sheets.
As for trackplans I initially fancied basing the layout on the M&SWJR section between Cirencester and Andoversford so making it double tracked and looking at Foss Cross and Withington as inspiration. I tried using any rail 6 to make a diagram but ran into issues making curves using flexitrack. Using settrack curves at third and fourth radius I got
This design leaves half a foot at the sides from using set track, but more importantly only leaves a foot between the end of the track and tunnel mouth, hardly enough for a long bridge and embankment leasing up to the tunnel mouth.