VM Cable BB is great
I have had VM's 20Mbit service since they introduced it to my area, and have never regretted it.. I always get the full 20Mbit speeds (until i get STM'd) and reliability wise, it's rock solid.
I did need to get a new modem tho as my old 3Com CMX (been Cambridge Cable, then NTL, then Virgin cable bb customer since they started in my area) would only go up to 10Mbit. I just filled out the form on Virgin's web site, and my new modem arrived a few days later free of charge. A single phone call to pair it with my account, and I was online at 20Mbit 5 minutes later. Nice.