Our group bought a pack of these:
http://www.ukflooringdirect.co.uk/Laminate/Florence_7mm_Laminate_Flooring_Classic_Oak_Plank_V_Grooved.html
Or something similar for twenty quid. Slot together on a kitchen table for a decent sized gaming surface. The boards are about 8" wide and 4' long, so you don't get a huge amount of extra width to your game, but you do get a lot more length (i.e. distance between you and opponent) than with a standard kitchen table. We even managed small games (4'x4') by laying them on a large coffee table.
We currently lay a mat on top for colour, but you could paint and flock individual boards. We plan to add some details for rivers etc. The nature of the set-up means that these features could be moved around fairly easily.
Will stack and hide under a bed, behind a door, behind a settee or wherever you need it to go. Fits in the rear of a focus if you need to take it to your mate's house.