VM are best in terms of speed (though not availability) but the installation cost isn't free I don't think. They also fight data requests (i.e: companies that monitor torrent seeds etc) that aren't court ordered while Sky doesn't (or didn't, at the end of last year).
Sky have greater availability and are cheapest of the lot, but their 'up to 20mb' line is never anywhere near 20meg, so if speed is important don't go with any of those 'up to xxx' deals. No installation cost with Sky though.
None of the others are worth considering really. They tend to be cheap and cheerful with throttling and bandwidth caps and that kind of thing. Facebook internet, basically.