It is a pretty common building material. I am sure there must be companies in the UK that sell insulation. If you can't get in in blocks, then perhaps sheets? I know you can buy 2" thick, foil backed sheets.
If there is enough interest, I can look into buying some and shipping it out. I know that you can purchase it through the web, but I seem to get the idea that you get it in 12"X12"X12" block. I think that something like a 12"X24"X6" block might be more practical.