I don't live in the UK, but I have shopped a few times from signmaterialsdirect on ebay. They also seem to have a website http://www.sign-displays.com/ but I have only bought from them through ebay.
I have used [www.cutplasticsheeting.co.uk] but they are quite slow and it turned out to be "Dibond Digital" which only has 0.2mm aluminium skin rather than 0.3mm.
