Hearing aids should be VAT exempt. This is because you don’t pay VAT on items designed specifically for a particular disability or condition. So regardless of diabetes your hearing loss , being a specific disabling condition, and the items to alleviate some of that condition, the hearing aids, are therefore exempt.
It can appear complicated. More info from here..https://www.gov.uk/guidance/vat-relief-on-certain-goods-if-you-have-a-disability