Dont even touch O2 Access! They are the worst people I have ever been with, where I was getting dial-up speeds at the evenings and all weekends, to the point where I couldnt even watch videos in youtube! Plus Net are not that much better either, as they heavily throttle you for any P2P or downloading you would be doing, so on the strength of that dont bother with them either.
I have heard very bad things as far as Virgin is concerned, but ADSL24 do sound quite good from the people that have tried it on here, and as they only have a 1-monthly contract, its well worth a try as you can just leave if your not happy with them. It seems for around £27 a month you get 60gb daily cap, and free usage midnight-9am weekdays, and all weekend!
If you just want to live with 50gb total monthly limit, then you could try Cerberus, who I am with at the moment. I get maximum download speeds all day, everyday, which is a breath of fresh air for me with the other people I have been with.