I read your message with incredulity and sadness in equal measure.
Essentially, and on the assumption that you have already disclosed your condition to your employer, they are or should be aware of the Equality Act 2010 and their obligations with regards to disability. Multiple Sclerosis or MS is a condition which is regarded as a disability from the point of diagnosis .
With regards to the meeting you attended, you have the right to reschedule it up to five days and should be given 48hrs before attending. If your advocate / companion is not available and your employer puts you under pressure to attend, that could in itself be a failure to make a reasonable adjustment, which they are under an obligation to consider. That is, to recognise that under the circumstances they should allow you time to prepare your case / response. They should also particularise in sufficient detail the conduct or in this case capability issues which they are considering. that is, how long is the absence, what dates etc, etc.
Their “ultimatum” (to attend the meeting) could well be direct or most likely indirect discrimination by applying a Provision Criterion or Practice, which puts you as a disabled person at a disadvantage, when compared to someone who does not possess the same characteristic.
They should have asked for Access to medcal records, in order to determine you current condition and the effect it has on you, day to day. Without this, or some other medical report dismissal would not be safe and could amount to an Unfair Dismissal.
So I would suggest the following
an email advising your employers that you regard yourself as a disabled person and that they are fixed with knowledge from te date they first became aware
That you will attend welfare meetings, but would like to be accompanied amd also you wish to discuss reasonable adjustments that they intend to take to support you at work.
Copies of any previous meeting notes where you have attended to discuss your absence.
Talk to your union rep or go to CAB or consult ACAS for advice on your employment rights
And dont attend any more meetings unaccompanied. Stress can excerbate your symtoms and your employer should not act in a way likely to damage or undermine the implied terms of trust and confidence written into every employment contract, and putting you under pressure to attend meetings, pretty much does that.
How long have you been employed and what is the job that you do, if you dont mind me asking?