VRDP29700 | Motor vehicles [items 2(f) and 2A]: parts and accessories
From HM Revenue & Customs · VAT Relief for Disabled People Manual
A business may zero-rate any parts or accessories with the first supply of an adapted vehicle. Supplies after this first sale, will however, only qualify for the zero-rate under item 2(h), if those goods themselves qualify as having been designed solely for use by a disabled person such as a replacement wheelchair hoist.
The following would not be eligible for zero-rating:
radios;
mobile phones; or
personalised number plates.