I now can't find the text about the 2 year grace period, so I'll happily retract that.
There are some Q and As
here on the CAA website. Close to the bottom is an answer about EU state issued commercial licenses being used on G reg aircraft...apparently just an online form is required to allow you validate and fly G reg. Admittedly "conditions apply", and I'm not sure exactly what those will be
Point taken about working rights though, that's another matter