I have been doing some digging in the voipfone website and found out, in the forum section, that they do do some porting, but only for certain geographical areas. The information is not terribly up to date since is a good few months old (April), so with a bit of luck they will have expanded the service by now.
In case someone wants to know more, info available here:
http://www.voipfone.co.uk/Porting_Codes.xls
and more in general here:
Number Porting From BT

Andrea

PS

As Matt said, it seems that nobody is as yet able to port numbers from NTL/Telewest, but only BT.

PPS

I have had another look at the forum and they have another list of all the dialling codes they can port from, including Dundee (WOW!!).
Entire list here:
Transferring (porting) your telephone number to Voipfone
I am not sure what the difference is between this list and the one I mentioned earlier on, but there you go.....