This is my specialist subject. It is really important that disabled people exercise their right to reasonable adjustments as it keeps them in work.
It is correct that your employers should know or ought (given the evidence) to have known about disability to entitle you use the Equality Act, both not to be discriminated against and get disability related reasonable adjustments.
There is no requirement to do so before application, so perfectly okay to disclose now and I suggest you ask for the changes as reasonable adjustments.
I would advise you write to your line manager setting out details of your disability and the significant impact it has on day to day activities (when not on any medication prescribed). Then set out what RAs you need and precisely why you need them, why it would be reasonable and any other relevant factors to support your claim (how others get flexible working, low impact on others, minimal cost etc). If you have any medical notes/reports that would support your application then use them.
Also asked to be referred to Occupational Health. Best to get the detail because often Occ Health reports are poor what changes are needed so best to give them good guidance. Ultimately the decision is with your employer, not Occ health but a recommendation by Occ Health is really helpful . in making the decision you employer has a duty not to be unreasonable and any decision to refuse has to be objectively justified (and the burden of proof is on them)
Do not let them treat it as a Flexible Working request, but a request under the Equality Act .
If they say no to the request (and you think it is wrong) put in a grievance and if turned down, do exercise appeal rights. You could pursue a claim to the ET but note time limit to starting Early Conciliation(which would be within 3 months less one day of date you think the RA should have been put in place after request ....do not wait until grievance process exhausted if longer)
Good luck!