if a screwfix/toolstation trade desk is near, then you will get it for £8.94. Bnq is much easier to get to for me as its 8 miles closer, and is open till 9pm. If you have a trade card, you get 2 months to pay, and if you have a trade discount card you get a bit more off.
http://www.diy.com/diy/jsp/content/trade/products.htm
The insulation deal is cheap, 200mm thick, triple roll, £5
The sheets of ply (I think its bbc grade) are ok on price, but again its more so its handy, as they cut it for free as you wait.
The end aisle deals can be hard to beat at times.
My plasterer only uses bnq warehouse as bnq stuff (all trade names) is the cheapest around, and he can go after work, and the place is still open.
The bulk bags of sand etc are again an ok price (a few ££ in it when your only buying 1), but they deliver it for free, when the order is over £50 and you pay with a trade card.
Bnq obviously cant compete with single trade bulk buying prices