Unfortunately, according to Mind:

"But your health and welfare attorney won't be able to refuse treatment for you if: You’re sectioned under the Mental Health Act in the future, and your treatment is prescribed by the responsible clinician or approved clinician in charge of your treatment at that time. Your attorney will have no legal power to refuse the treatment. This is except for ECT, which they can refuse even in these circumstances. ..."

https://www.mind.org.uk/information-support/legal-rights/men...