It seems to me that the easiest route for at least Roll-Royce and the UK bits of Airbus would be to just completely ignore the "new, improved English CAA"
If these organisations maintain EASA registration - if necessary through their German subsidiary and/or French owners - they can still produce EASA-recognised airframes, engines and parts, which will be able to be fitted & maintained by EASA-registered MROs & airlines.
If the English CAA was then directed to get all jingoistic and not recognise EASA approvals, that would put the UK based industry in a pretty pickle, but the rest of the EU would probably shrug & walk away