I'm getting about 4mbps from O2 – when I run a speed check. Which was a marked improvement from Zen Internet. The line length from the exchange is key to the lack of signal and unfortunately we live far away from the Bermondsey exchange.
It is possible to improve the performance by upgrading the kit on the exchange, I think that's why the newer O2 kit increased the speed I could achive over the same line.
It's all a bit suboptimal, sadly….