On 18 December 2020, Ofgem approved the BritNed Access Rules 2021 that came into force on 1 January 2021. This can be found on the Ofgem page here.
On 21 December 2020, ACM approved the same BritNed Access Rules. This can be found on the ACM page here (in Dutch)
The letters of approval, and the BritNed Access Rules 2021 can be found under Participants Portal -> Key Links and Documents -> BritNed Access Rules and Harmonised Allocation Rules. It is accessible via thislink
The Approved BritNed Access Rules 2021 can be downloaded directly here (PDF)