definitely a London thing although obviously heard elsewhere too. It's wrong wherever you hear it though.
People who say it don't "hear" it as wrong so it's difficult to stop yourself from saying it if you were brought up with it as the correct form. As Coffeeshoppe says, it definitely doesn't sound correct to most people but if you were brought up with it you just think that is the way you conjugate that verb. It's a shame because you will definitely be judged on it.