Good deals are a b!tch to get now,
Internet : 2 cheapest are Dealdrivers and Broadspeed..
Dealdrivers do not have any more cars available now till after the 1st Sept, and will only accept orders on the price increase from this date. Broadspeed are a little more expensive but wont give you as much of a 'deal' if you want scrappage.
Verdict : Dealdrivers can I disagree, Broadspeed would be my choice, should still get 10% discount from them if no scrappage.
Dealers :
I've never seen any dealer so far match the Internet for prices, some come close, from what I gather top discount seems to be around 7% with an average of around 5%. Then that 5% you were saving will be taken from your hand come 1st Sept, you will from then on be paying full price for the GTI. Then come the extra 2.5% VAT added 31st Dec.
Verdict : For the love of god, buy now !