A good bet for plastics is directplasticsonline. http://www.directplasticsonline.co.uk/

The shipping is a bit steep but its worth it. Its also well worth checking out their clearance section as thats stupidly cheap and theres onften some good bits in there.

Id advise HDPE as the the best plastic for a bot, if you want somethign stiffer (but more birttle) for structural parts polycarbonate is good, but quite expensive.You should be able to build a basic ram bot from just hdpe.