There are the other things to consider with VM though, Phorm being one of the big ones. 12 month contracts another, at least with most ADSL providers you have 1 month contracts, OK your BT line is 12 months but you need that anyway for the service.
If you're a 'heavy downloader' I would think very hard about the STM system and whether you can live with the restrictions, if so then you should get a decent service out of it - especially with the bundle deals you can get very good value for money.
VM's customer support is probably no worse than Be, which is ****e, I've had the misfortune of having to call them up a couple of times and just ended up getting frustrated that I could never understand the person I was talking to and they were more intent with just fobbing me off with excuses than admitting there was an issue or actually trying to solve the problem.
Zen on the other hand are brilliant, full speed all the time, 10ms pings to anywhere within reason, support from the UK by people who ACTUALLY know what they are talking about and don't just read off a script. But I pay through the nose for the service (£45.50 a month inc. the line rental from BT) which is by no means a cheap connection.
If I had to chose between my Zen connection and cable then I know I would go for Zen as at the end of the day they are a premium provider and they deliver the service that I expect, and when there is an issue (which there rarely has been) I know it would be sorted out very quickly.