I am new to this forum and have been reading with interest about the Megane 225, as I am looking at possibly buying one.
On the matter of a good rolling road in London, you won't do better than speaking to either Steve or Steve at Track & Raod in Rainham, Essex. They are on 01708 869189. They have tuned my old MK2 RS2000 which runs on twin carbs and a turbo, and are ace with modern turbo'd cars. Their rollers are good for 800bhp!!
Oh, and they're nice chaps too.
If you call them, tell them the recommendation has come from me, Per.