Including those in the UK “without Permission”
https://www.gov.uk/guidance/nhs-entitlements-migrant-health-guide
There can be no charge made to an overseas visitor for the diagnosis or treatment of coronavirus (COVID-19).
All overseas visitors, including anyone living in the UK without permission, should be aware that:
1. No charges apply to testing for COVID-19, even if the result is negative, or to any treatment provided for COVID-19 if the result is positive or up to the point that it is negatively diagnosed. The same is true of most other infectious diseases.
2. NHS trusts have been advised that no immigration checks are required for overseas visitors that are known to be only undergoing testing or treatment for COVID-19.