If you do decide to come, get onto the HK public system asap you arrive. Public system is WAY better than private for anything serious like MS. Even if you had full private coverage, you should still go onto the public system for this. Be very careful about trusting private system specialist doctors for chronic long term illnesses like this. (Private sector GPs, PTs, OTs, tend to be OK)
Use private insurance only to top up for things like scans, GP check ups (just make sure they're not specifically for MS).
Negotiate - the top expat executives here can be offered health insurance packages that cover pre-existing illnesses.
It's your decision whether to have an adventure and stretch out of your comfort zone while disease under control. Top UK teaching hospitals + a really good GP will probably offer you a better all round care experience. But if all's quiet for now, I can see why you might want to go for this. If you do come, make sure you guys have exit plan in case anything here triggers flare and you want to go back to UK.
Presumably you've worked out likely triggers for flare up, if any? e.g if heat or heavy air pollution is going to cause a flare, then .... um....
Ask your UK consultant if s/he has any contacts with HK public system specialists, and if s/he can (a) provide you with direct referral (b) check if key medication you need is provided free or at subsidized cost here. Senior/experienced doctors in HK used to train in UK for MRCP and other advanced qualifications, so chances are the top doctors here know counterparts in UK, assuming they're still working in the public system.
Here's link to what's covered fully, covered but you pay them out of pocket (still massively cheaper than buying at commercial pharmacies), or which you have to buy yourself from commercial pharmacies (have found mark ups for drugs tend to be 8-12X cost).
Drug Formulary
Happy to provide GP, PT and OT referrals once you get here - all private sector, see them as supplementary to main public sector care.
Good luck, and keep us posted on what you decide, and how we can help!