Website     Reliefs from VAT for disabled and older people (VAT Notice 701/7) - GOV.UK

It is down to the supplier to ensure that the person and the equipment meets the requirements laid down by the government.

Find out about which goods and services for disabled people and people aged 60 or over that you should apply zero or reduced rate VAT.

Generally, the client is required to visit the supplier to discuss and choose the equipment required and specified in the OT report. The client is then required to sign an exemption form which is kept by the supplier. 

If bought online the supplier will request an email from the client to confirm their situation, which is retained proving the client is VAT exempt. Sometimes the company will send a form for the client to sign. 

The client doesn’t have to be registered disabled; however, they must have a confirmed diagnosis for a condition that impacts them i.e., chronic long-term illness such as diabetes or COPD.

See Form 24 - Relief from VAT on Goods & Services for disabled persons