bt estimate 23.8-33.2 download and 5.8-7.1 upload
Likely to be interleaved (error correction, increasing latency, possibly higher than ADSL2+), speeds would be a bit faster on Sky than on Zen & Plusnet. Location also affects ping, if you're close to London then you'll get lower latency on Sky and Plusnet (not sure about Zen's network), compared to up north.