First, document your attempts to withdraw, making sure to keep all conversations you have with the centre on paper (otherwise it's their word against yours). Second, contact the centre again and keep the evidence; threaten to sue during the conversation if nothing can be done, making sure to tell them Edexcel have said it is possible for you (centre) to withdraw me. Then contact the exam board again. If all else fails, seek legal advice and consult an education lawyer, if necessary (and if you have the time).