Thanks. I did not know about the 28 day guarantee. This means I can at least see how it is, and do a full cancel if I have problems.
I'm around Roehampton, not too far from Wimbledon village, and did this exact thing a couple of years ago. VM were actually very good in terms of getting the appointment booked in and the service installed quickly, but jitter was horrible.
Ended up going with ADSL and am now on FTTC. There was a bit of lead time between ordering a BT line + ADSL and the OpenReach appointment so it was nice to have a 'proper' internet connection in the mean time.
VM didn't really question the reason why I cancelled, it was very straightforward. A guy came to my workplace to pick up the Superhub, even!