The best ISP in London is really AA ISP http://aa.net.uk/ it's the ISP's ISP. Run by network nerds that really love what they do.
They are the only ISP that has no filtering whatsoever whereas other big ISP's have porn and torrent and other questionable censorship filters they do not do any of that.
They also provide full IPv6 (and up to /48 address reservation) to every customer by default.
Speeds up to 80Mbps (same as every other FTTC provider). They have a bit weird but ultimately very transparent pricing.
I have had them for over a year and it's been a breeze. Their support is also very competent.