Get away from them as soon as you can, managed to get my parents off BT last year, best thing I did. We were actually on the 21CN ADSL2+ trial and the service was just as bad as their ADSL product, sure we had a faster connection but you couldn't make any good use of it, they throttled traffic to death if it wasn't HTTP traffic (they were running the trials with the aim of deployment in October 2008, doesn't look like that happened). They also are very expensive, supply a crappy locked down router and have involvement with PHORM, not a good company to deal with.
With regards to the O2 latency issues, yes they exist, I suffer from them time to time but it's because I'm stuck on their Access package which means you are connected using BT's equipment rather than O2's. There is very little O2 can do about it apart from installing their equipment in the BT exchange.