Hi Clokisk.
I’m assuming disabled travellers use the same companies as other people. Just have to disclose detailed info on the disability. Sometimes that may result in increased fees or being declined for certain aspects of the policy but I’d start there.
https://www.consumer.org.nz/services/travel-insurance/guide
Kiwis have reciprocal medical cover with the UK. https://www.health.govt.nz/new-zealand-health-system/eligibility-publicly-funded-health-services/reciprocal-health-agreements
https://www.canstar.co.nz/wp-content/uploads/2019/03/Canstar-Travel-Insurance-NZ-2019-Singles.pdf Comparison travel insurances in NZ in table form. UK scroll towards the bottom. There are different options if you’re after travel insurance for one trip or multiple trips etc.
Hope that gives you a starting point?
Forgot to report back but my main claim for reimbursement was eventually successful from the credit card company. I had to be my own detective to some degree and things took a looong time to sort but a good outcome.