Thus far, I've checked around and Tesco currently have it priced at £37.97 in-store. This is the same price as Morrisons. £28 if you buy it with 2100 Microsoft Points at Tesco.
However, a clerk in ASDA informed me (probably against her regulations, based on how confused she was looking through the computer) that the standard edition is penned in at £34.94, as I recall. To date this is the cheapest I've found it.
Aparently going to be 37.99 on its own like Tesco.
Gamestation have it priced at £37.99 as a competitive price point, but have a trade-in deal going; bring in any two XBox 360 games (and I mean, any) and get an extra £5 trade-in credit. My tip? Gamestation are selling Rockstar's Table Tennis and a few Pro Evo titles at 98p. Purchase two, and trade them in on the day, and save another £3.
As I am aware, GAME have a comparable deal going, being under the same parent company.
Currently HMV have it priced at standard RRP of £39.99, but they're giving away a free 800MS point card with each pre-ordered copy. Moreover, they have an excellent trade-in deal if you have either Red Dead Redemption, Hawx 2 or Mafia 2 to trade in against it; bring in one of those games and you'll get Halo Reach for £14.99 (meaning £25 trade in credit for the above titles). Please be aware HMV require a £5 deposit for preorders whereas GAME and Gamestation do not; this is obviously taken off the price of the game when you purchase it on launch.
So far Argos still have £39.99 on their system with no other incentives.
'Thanks to SigmaLongShot @ GameSpot*