Cheapest ive found online and shopto are the best for delivery, usually get games a day before release. Doubt that will be the case with MW2 but your guaranteed delivery for tuesday !
Personally i think this deal is hot, it beats game's deal where you trade in MW1 and get MW2 half price since your guaranteed a copy with shoptp and Game hasnt even confirmed their price yet ! Not only that if you still own MW1 you could always stick on ebay to end this weekend and your guaranteed around £23, post the game and pay your ebay fees your getting around £21 for MW1 which aint bad !
Theres the possibility that Morrisons / Asda, etc will get a few copies of MW2 in and sell them at around £35 but no one knows for sure + you probably wont get it on release date from supermarkets.