This SS has not yet been added to the PRA Rulebook. It is available on the Bank of England website, here.

https://www.bankofengland.co.uk/prudential-regulation/publication/2015/solvency2-group-supervision-ss

Related links

Group Supervision
PS6/22 - Solvency II: Definition of an insurance holding company https://www.bankofengland.co.uk/prudential-regulation/publication/2021/september/solvency-ii-definition-of-an-insurance-holding-company