Are the COC documents still accepted in the UK after Brexit?

For the time being, yes (as of September 2022). COC papers are still accepted in the UK: You can find the details here: https://www.gov.uk/guidance/certificate-of-conformity-for-new-vehicles

EU certificates of conformity remain valid for vehicles sold in the UK for the time being. The current requirements are in the retained EU Regulation 2018/858, Article 36 (1).