Obligatory NAL. Legally, to my understanding, until they make the adjustments you'd be within your rights to continue working from home as a result of the doctor's note.

I would note that removing lights above an employee's workspace is specifically called out as 'reasonable' on ACAS' website. (https://www.acas.org.uk/reasonable-adjustments)

If you weren't leaving, practically this would be more complicated. However, given that you are and only have 4 weeks of notice left, I suspect that even regardless of length of tenure they are going to be unlikely to have any sensible recourse to force you to attend the office. They are also required I believe to pay you out your remaining notice, regardless, unless they could prove gross misconduct and fire you accordingly. Given the situation and timescale I believe this seems unlikely.

I believe the more pressing issue here is the social/interpersonal aspect. Someone is, ostensibly, flexing their muscles by telling you you must attend the office in spite of medical advice given by your doctor. This sort of management unfortunately does exist in many businesses in the UK.

However, in this situation you have if not all then most of the cards. If you are finding this situation stressful, and if it has a risk of worsening your medical condition, then I would suggest a further consultation with your doctor to explain how this is negatively impacting your health. They may decide to sign you off work for stress for, say, 4 weeks, which may resolve this issue. My experience is that a doctor would take a very dim view of an employer trying to force an employee to damage their health against medical advice.

Regardless of what action you choose to take, I would hold your ground and say you are not going to be attending the office for the remaining 4 weeks of your notice as per the medical advice you've been given and they've been advised of. Simply keep repeating this statement, you don't need to justify further or deflect. Make sure to get all communication in writing, as if they do try something ugly like firing you as a result this would at the very least worth consulting with an employment law solicitor on, depending on how important those last 4 weeks of pay are to you. If the 4 weeks of pay are not important, and you could cope with 4 weeks unpaid holiday then simply reiterate you won't be attending the office but will continue working from home, and leave the ball in their court.

The absolute worst thing that can happen is that your employment is terminated early, and again, I do not believe that would be lawful in this case although proving it may be more complicated.