> I'm looking to buy some rigid fibreglass in the UK <
Also look for rigid mineral wool and rigid rock wool, which are basically the same thing as rigid fiberglass. Knauf is a large manufacturer of these products, and they have sales offices all over the world. Here's their web site:
join this site, check with Paul (here's his Studio Build Diary) - he's a bit over the top, but if he can't help you find things they probably don't exist.