Just because they can't legally withdraw the offer or fire you based on the form doesn't mean they won't, and it doesn't mean it won't be a problem if they keep you. I would decline to fill out the form. Just leave it blank, sign it, and turn it in.

I've dealt with various forms of discrimination as someone with epilepsy. Disclosure is an individual choice, but in my experience the best thing is to not say anything. I do tell people, but not when starting a job. Some people are ok with it, others can't handle it.

If no one at the company will ever see the form and they send it to the NHS or insurance, it's probably ok.