You could migrate to pay as you go and go back on contract when you get back, you will need to top up evey six months to keep the number active though. Or you could to a £10 a month simplicity deal.
From what has been posted previously, I believe you have to make a chargeable call every 180 days and top up every 999 days but I'm sure someone from O2 can confirm whether or not that's correct.