If you're on contract with o2, you will get a call from them every 4-6 months asking if they can help, and usually offering to get you onto a different pay plan to suit your usage better. You will definitely get one before the expiry of your current contract.

About 50% of the time this does not extend your contract, and about 50% of the time it does. Always, always ask "does this affect my contract" before verbally agreeing to any changes to your plan, and state "I do not want to change plan if it affects my contract" (unless it's an awesome offer, of course!)