I found a great company on ebay for all forms of Kingspan - sold as seconds but all the batches I have had so far were spot on with no damage at all.

much cheaper than the merchants and they have regular auctions

http://myworld.ebay.co.uk/secondsandco/

or you can buy direct

www.secondsandco.co.uk

interpretation of the regs vary from council to council and even between inspectors - in my area the thin foils are not approved at all, but they accept 100mm of kingspan between the trusses

edited to say on new builds you have to double board the ceilings between the first and second floors and fit fire doors on all the upstairs exits, but on refurbs/conversions you only have to fit pirco door closers to the existing doors upstairs and no double boarding is required, and have a means of escape window in the roofspace.