The Health and Safety at Work etc. Act 1974 says employers should take reasonable steps to ensure that workplace factors do not cause or exacerbate poor health. This includes assessing and monitoring stress factors that may impact on you in the workplace and trigger migraines.

Additionally, the Equality Act 2010 requires employers to make reasonable adjustments for employees where working conditions put you at a disadvantage aka trigger migraines. Migraines aren’t automatically classed as a disability but your GP can give you advice on if they think it would be - if you’ve had them for years and are expected to have them for the rest of your life and they impact long-term on your ability to do day-to-day activities then it could count.

WFH and adjustments to lighting are pretty common reasonable adjustments for people with migraine related disabilities.

I would reply and quote the above acts explaining that you have requested to WFH due to disability related migraines and enquiring if they have made any changes to the building lighting to accommodate your disability related reasonable adjustment. From their perspective the implied threat of a tribunal case isn’t worth making you come in for 4 weeks.