The game isnt officially released until the 11th in the UK. Simplygames are flogging the game cheaper because it doesn't come with the Casino DLC or any bonus cars (Which are worth up to £11 more) - Amazon and other major retailers are.
They have also been selling the game for the past 2 weeks already. Which has caught the eye of some of the UK watchdogs and authorities. I also suspect, like Shopto, SG's games are not produced for the UK market ;)
Id rather stick with the fully legit version for £2.50 more, get all the DLC free and only wait a few more days thanks.