This can happen from time to time and it’s not a pleasant thing to go through. Unfortunately, there are  people out there that take it very personally if you choose to leave them. While this is often a clear case of unprofessionalism, it can be a pain to try and get things sorted. We would suggest that you start by writing to them with a formal request, we can help with this of course. Then you have the option to start a domain dispute with Nominet or take legal action. We can help point you in the right direction.