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/2013/counterparty-credit-risk-ss

Related links

Counterparty Credit Risk (CRR)
PS22/21 - Implementation of Basel standards: Final rules https://www.bankofengland.co.uk/prudential-regulation/publication/2021/october/implementation-of-basel-standards