Better Metal Stores in Alfreton are a good source for small amounts of sheet, bar and profiles in most materials (metals and plastics) but they are probably pricey, but a useful supplier nonetheless.
Proops Brothers in Fleckney have 3mm sheet 250x500 (mm) for £35, but I don't know the grade. They also have a lot of cheap (and cheerful ?) bits for small engineering.

TTFN
Rob