As has already been said, it would be discriminatory to refuse the loan on grounds of MS.
But, to put it in context, it's you, the borrower, who would be taking the risk.
The whole point of a mortgage is it's secured on the property, so that if you default, the lender can repossess it to recoup their money.
You will not be granted mortgage/income protection insurance for MS, or any condition arising from it, although it will probably still be possible to insure against other unrelated health issues.
So that would mean if you did become unable to work through MS, it would be a risk you'd opted to take, and you wouldn't be able to claim on insurance. If you were unfortunate enough to have to stop work from an unconnected illness (e.g. cancer), you'd be able to claim in the normal way - but they'd need to be very sure the "other" illness was the real cause of your stopping work, and that it had nothing to do with your MS.