I can recommend Sky Fibre Unlimited Pro, I've had it for about 11 months and there hasn't been any slow downs, I'm not sure if it would been the same in London though. I get full 80/20 sync.
Here are some speed tests:
I also sometimes use a VPN (on my router), it's the OpenVPN type and it adds only about 1ms of ping and loses about a 'meg' or 2 (because lzo compression is used I suspect, not because they don't have enough bandwidth) of speed if using the London VPN, it's very good.
Edited by CiscoGuyUK (Sun 28-Apr-13 18:40:55)