If you have been with them for 2 years already then I guess it would be difficult to get out. I would suggest doing a freedom of information request, FOI, and ask them for an unedited audio copy of the original call where they sold you the contract. By law they have to keep these recordings (but for how long I don't know). They have a month from your request to provide it. This is the evidence that you need if they lied to you about being from O2 and if they failed to mention that the contract was a non canceallable 3 year contract. If the recording shows they misled you then they most likely will ignore your request or try to edit the recording (according to others). You then could consider telling them that their failure to provide the recording proving it was a 3 year contractwith O2 allows you to withdraw after 2 years.
The option many take is to go through the Ombudsman and they do this on your behalf. I guess you would say that you thought or were told it was a 2 year contract with O2. The ombudsman are well aware of Plan.com and it may be your best option.
However all of the above takes time and is a hassle. You may be better off just seeing the contract through if there is only 1 year left on it. Make sure that you inform them WELL in advance of the contract end that you will not be renewing and send by post and email, keep posting records and copies, and just make sure that they know and you can prove it. Then as soon as your contract is up cancel the direct debit with your bank before they have any chance to take more money from you. There have been reports where they keep taking the money from the bank every month even though the contract has ended, so make sure they can't do this. Some people have also said that Plan.com still went ahead and renewed them on to another 3 year contract without their permission, so make sure to stop that happening in advance of the contract ending by clearly telling them and cancelling the direct debit as soon as the contract finishes.
Good luck
