I've got ADSL with Andrews & Arnold and they've been excellent. Whenever there is a problem all I do is give them a quick call and they normally sort it out within an hour. All other times have been due to BT screwing up somewhere.
They also do colocation but it's £100 for a 1U server with 100GBit a month allocation. They've got a 100Mbps link to the outside world which is better than the BlackCatNetworks one however.

- Trevor