Except that after tax and council tax, and energy bills and phone bill (internet and mobile), there is no way you would have that £6.6k in London. It'd be possible in another part of the UK maybe, but not trivial by any means.
Council tax doesn't increase with the number of people in a flat (>1), so when you're sharing the price for that goes down dramatically. You could easily reduce your spending in other categories as well if you wanted to spend more on rental (i.e use busses rather than the tube).
Gumtree currently lists 19k flatshares below the £125/week mark:
http://www.gumtree.com/search?property_room_type=&property_t...