I guess the question is what you mean by "open banking". Initially, in the UK, that phrase referred to the FCA's implementation of the PSD2 requirement for banks to allow a secure mechanism of access to third parties. I think that this definition of open banking has already regressed post-Brexit, from the absence of passporting. UK firms and banks are no longer able to interoperate with EU firms and banks, and PSD2 no longer applies to them.
Another definition may be domestic API access to bank accounts, which I agree will continue to be policy in the UK. It won't be PSD2 open banking, though.
Another definition may be domestic API access to bank accounts, which I agree will continue to be policy in the UK. It won't be PSD2 open banking, though.