CHECK is the name of the scheme under which NCSC-approved companies can conduct authorised penetration tests of public sector and CNI systems and networks.

CHECK is the umbrella term for the NCSC approved penetration test companies and the method in which they conduct a penetration test. Companies providing CHECK services will do so using staff who hold NCSC approved qualifications and have suitable experience, and pen tests will be conducted using NCSC recognised methods. The subsequent report and its recommendations are produced to a recognised standard.

https://www.ncsc.gov.uk/information/check-penetration-testing