Just recently bought an iphone for myself as works out a lot cheaper buying the phone and sim card separtely. You arent Tied in and can cancel when you want. £15 per month and you get unlimited texts, 1gb data and 250 minutes which can be rolled over if not used.
Tesco are teamed up with O2 and seems to be the best sim only card around in my opinion.
Ive saved myself about £200 doing it this way.