If you are on o2 Pay as u go, you can ring customer care and explain you are interested in upgraing your handset. They will average how much you top up per month and offer you various hansets. Some great deals available!!
My brother just had a Samsung E900 for free on pay as you go and he is keeping the same number. Well worth a phone call.
Tel: 08705678678 from landlines or 4445 from your 02 phone.