I don't know the answer to this, but I know someone who had a contract with 9mths remaining bought out by Phones4u as they were going onto a new 18mth contract of £35 per month. The previous contract was £30 per month (if I remember correctly).
I think it would depend on the new contract you take out as in is it a big contract (£30+ per month), is it with a handset they're struggling to shift stock of? And also, what's the commission for them like from the operator for getting a new customer on board?
I'd guess that you wouldn't be able to keep your existing number but I could be completely wrong.
Best thing to do would be to go into the store, and make an enquiry at first - test the water. If you hover around the handset stands and they come to you quickly, that'd usually mean they're pushing for sales which is likely as people will be spending less after Christmas!
iPhone 4 16Gb Black
(Still got the Blackstone for the iGo8 GPS - until I can afford one for the iPhone at least)