If it's actually cable you need to contact Virgin Media.
If it's just a BT line you need to contact BT to get your line activated (if you're the first occupant they will charge you £125), then once it's been activated wait a few days (possibly a week) before it will appear in the BT ADSL database all the ISPs use. Then just place an order and you will be given an activation date (usually about a week later).
Apparently it's also possible to get broadband activated at the same time as the phone line, BT will probably try to sell you their own to do this, but if you phone other ISPs they may also be able to help you arrange it.
Slighty off topic, but I was wondering what the range of the signal from John Harvard HotSpot is. Probably wouldn't reach the 200 yards to Empire Sq. You often see people outside JH on sat afternoon or sunday with their laptops presumably taking advantage of the signal.
Wonder what will happen when WiMax with a range of 80km arrives.
SE1 must be dotted with HotSpots.