You canb always have it unlocked for �10 and then it will take any SIM card, I do this with all my phones. If you ask in any phone shop, someone will know a place where you can do this.
Contact your existing supplier and ask for a PAC code (Porting Authorisation Code). Give this to your new supplier along with your mobile number and they will transfer your number for you.
When I moved my number to Virgin I was told only 3 couldn't retain old numbers, but may have fixed that problem by now. As a rule it's usually easier than keeping your number when switching to or from BT at home.