I reckon it won't be set in London. I'll be a completely new city, one we haven't played in before. GTA's 1 & 2 established the principal GTA cities (LC, VC, SA) and GTA3, VC, & SA were Rockstar finally realising their dreams of rendering those original cities in 3D. GTA London, GTA Advance, LCS & VCS were pretty much just add-on packs to the main games so they can't be counted. R* will take the franchise forward by creating a new city, but judging by previous tradition it will be a thinly-veiled parody of an existing metropolis (usually in the US).