Sorry, no, if you are honest (which you must be) about having had optic neuritis, and you have ever reported ANY neurological symptoms to any medic, I do not think it will now be possible to obtain cover for MS - or any neurological illness.
If you decided to apply without mentioning it (not that I’m suggesting you would), and were later diagnosed with MS, and tried to claim, they would go through your medical notes with a fine tooth comb, and find out you’d had ON, which you’d neglected to mention, which would invalidate your policy anyway.
Sorry to be the bearer of bad tidings, but a pretty fundamental thing with insurance is you can’t take it if there are already warning signs the insured thing might be about to happen. You’ll either be refused outright, or the premiums will be so high as to be prohibitive.
It might be possible still to get cover for other, non-neurological conditions - at a price. But I doubt they will do even that, if you are still under investigation for an as-yet unknown condition. All they will see is that you have a health issue - not yet clear what, or how serious. I don’t think anyone is going to offer you critical illness cover in those circumstances. It’s a bit like trying to buy fire insurance when you’ve already smelled smoke.