I had exactly the same problem with Orange.  I received a customer
service call shortly after connecting my new phone and was offered the
Orange care for £5 a month.  The bloke never told me once that once I
had accepted this "Optional" insurance that I would be stuck with it for
12 months, nor did he mention the fact that the excess on Orange care
was now £15 compared to £0 which is what it was going up from.

I found that screaming and shouting and basically carrying on alarmingly
on the phone finally got them to remove it from the account.  I quoted
things like how can you make "Optional" insurance mandatory, especially
when they don't tell you that by taking it, you're then stuck with it
for 12 months.  Shout at them, carry on, ask to speak to supervisors and
refuse to get off the phone until they put you through to the insurance
department.  This is what finally got them to remove it for me 6 months
on into the contract, I think they were so sick of hearing me carry on
about it they just removed it.  Also threaten to write to the insurance
ombudsman and also to GISC (www.gisc.co.uk).  Every complaint GISC
receive and uphold they fine the insurance company a minimum of £500.
The 2nd rule under GISC is that if a customer asks to speak to a
supervisor, they must be put straight through, not offered a callback,
not fobbed off and passed about, they have to be put through to a
supervisor otherwise they have just broken the 2nd rule of GISC which is
an instant fine.

Hope this is of some help!

jeff_fox@despammed.com had a brainwave and did spout the following:

It's down to P4U for not explaining the T&C's, but they could argue that
it's down to you for not reading the T&C's.

Either way you'll be paying £10 + £5 per month to cancel your contract.