TheQ reckons you'd need 19m in OO
That's 1444m in real life
Or 4873.5 feet
4873.5 feet/30 feet = a scale of 1:162.45
Which is close to US/European N-scale of 1:160. But you'd have to build all the rolling stock yourself as, as far as I know, none is made in that scale for GB railways.
UK N-scale is 1:148, so it would be too big.
Of course, you could turn the layout on to the diagonal of the room, which would give you a max length of 39' 8" which would be just about do-able in UK N, and even make it a triangular roundy-round with fiddle yards next to the wall.