I don't know the laws in the UK, or what the consequences of not paying would be. I would think that she would have to sue you to force you to pay. She might not want the hassle of that. So if you write a letter saying that you are not paying and why, she might end up dropping it.

You could be proactive and file a complaint against her, either with the agency where she works or maybe from the consumer's association: https://www.gov.uk/consumer-protection-rights . They might be able to help you fight the charges.