Simulator needs to be approved by CAA (separately to its EASA approval) and the examiner needs to either have a UK licence or have a UK FCL.1000(c) approval based on their EASA licence. We have done this in reverse since Brexit for EASA licenced customers in our UK located sim.
Actually above is not quite true. If the EASA sim certificate was issued before we left EASA and is still valid, then it is accepted by the UK CAA. However I don't think this helps as they have probably revalidated them at BAA since Brexit