Interchange in the UK remains capped at the same 0.2% for debit and 0.3% for credit rate that applied in the EEA prior to Brexit. What changed was that this cap only applies when the issuer, acquirer and merchant are within the UK. Therefore, UK-EEA transactions are no longer covered. [1]

[1] https://www.barclaycard.co.uk/content/dam/barclaycard/docume...

The UK didn't scrap the Interchange Fees Regulation; but as a matter of law there became two separate IFRs, one covering the EEA-cards-in-EEA and one covering UK-cards-in-UK

(To put it bluntly, a big amendment was automatically applied to most UK laws on exit day replacing EU & EEA with UK; and of course in the EU the definition of both of those words changed)