Have you looked in Scotts tower bridge road, they used to have a large shop in rotherhithe and used T.B.R for offcuts but the bigger store burnt down..it is not big, go and see you may find a magig carpet!
Slight bias here as I work for them, but John Lewis carpet is surprisingly good value for money (so I'm told by people who have bought it). The floor coverings department at Oxford Street is a bit tucked away at the moment but it's rather large. There must be a reason John Lewis are growing their market share of floor coverings, anyway.
I used a carpet shop on Tower Bridge Rd a few months ago, can't remember the name but it's very close to the Bricklayers Arms roundabout. It's far cheaper than Carpetright on OKR and the quality was very good - wool, decent underlay etc.
whilst their prices are good I find Scotts on TBR rather un relaible. On more than opne occassion have waited for them to turn up to measure and give a quote. When I commented about their first no show I got a shrugs of the shoulders and no apology. A second no show means Im still looking for a local supplier of carpet.