They keep going on a rolling basis, you have to give 30 days notice to cancel. You can change your options by giving notice, to take effect the next month.
As long as you're happy paying for a handset you'll have no issues.
I've moved my iPhone to a 1 month £20 tariff and everything is spinky but I know if I want an upgrade going forward it will require signing my life away for X months (if not years).