Some FIs have reported difficulties when revalidating their FI certificates whilst choosing to exercise Class 2 medical privileges with their CPLs - i.e., with an expired Class 1 but still valid Class 2.
In some cases it was "Computer says No" at Gatwick; however, after AOPA recently pressed the point, the CAA has kindly looked into the matter and the situation has now been resolved:
It is confirmed that amendments to a professional licence would not require a valid Class 1. A medical must be valid but a Class 2 would be acceptable.