The CAA is at least doing something in preparation for Brexit: On LinkdIn there are adverts for Part 21 Design & Production Surveyors to form a new section for the UK - I assume BCAR solutions for UK designers, manufacturers and test houses...and there are quite a few of those to convert!
I would assume that the CAA's initial response, once the future is confirmed, would be to send all UK LAE's a letter explaining what to do next - and attempt to avoid a panic driven mass conversion crisis (again).