Answer - when not providing health care services, as a registered health care professional.  Seems straightforward enough but, as with many things VAT related, uncertainty exists. When registered health care professionals provide services using their professional training and knowledge, their services (with limited and specified exceptions) are exempt from VAT.  HM Revenue and Customs (HMRC) state services of a health professional are exempt from VAT when;

  1. The services are within the profession in which he/she is registered to practice and
  2. The primary purpose of the service is the protection, maintenance or restoration of the health of a person.

The exemption also extends to the charge for bandages, drugs, medicines or prostheses, administered or applied to a patient in the course of exempt medical treatment.

The provision of Covid testing kits, privately supplied for self-administration by the patient should therefore include VAT at the standard rate unless the provider has an annual turnover of less than £85,000. Given that travel to many places involves mandatory Covid testing it's hard to imagine many businesses with a turnover below the threshold, so why is VAT not being charged? There is no doubt the tests must be analysed by suitably qualified technicians and perhaps this is where such businesses believe the supply is being provided by a health care professional. Even so, the service is one of health screening rather than protection, maintenance or restoration of health and is still liable to VAT, whether the service is carried out by a registered or unregistered professional. 

I've no doubt HMRC will be looking closely at this activity and sanctions will follow. More clarity is also required so that all businesses in this world are trading from a level playing field.