Beware unclear O2 messaging!
When it notifies you about its hike on its airtime price hike - you've a RIGHT to leave penalty free within 30 days of notification.
Yet it also says “If you've a device plan, you'll still need to pay this off in full”. That DOESN'T mean you have to pay it all now, you can keep paying for the handset monthly with it, but move your Sim (ie airtime plan) elsewhere.