Where to get plastic card? UK

Most good model shops should carry plastic card such as net merchants

linkname http://www.netmerchants.co.uk/

Or you could get hold of the hobby's annual they have scratch building supplies.

Hope that helps.

Or go to any store that sells plastic "For Sale" signs and the like. Same stuff, but a lot cheaper. The only drawback is that I've only found it in three thicknesses (this was years ago) but it works great.

Hannant's, The Aviation Hobby Shop, Slater's, Squires Model & Craft Tools, Hobbies catalogue, Hobby's catalogue; all I can think of, at the moment.
Edgar

try thr plastic that seems to wrap everything these days. it is usually clear but does come in a bunch of thicknesses and should be plentiful with the holidays comin up.

If you're in the North of England, the Monks Bar model shop, in York, stocks it, some in quite large sheets. Sorry, forgot until today.
Edgar

generally who sells the plastics to do signs have plastics like styrene acrylics in many thikness

and are the cheapest materials.