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/credit-risk-mitigation-ss

Related links

PS14/19 - Credit risk mitigation: Eligibility of financial collateral https://www.bankofengland.co.uk/prudential-regulation/publication/2019/credit-risk-mitigation-eligibility-of-financial-collateral