Thanks guys for your answers. I am actually working for a TRTO in Switzerland and I was curious how it is handled in other countries as we often get inquiries from pilots who want to have their Copilot restriction removed. I guess the easiest way is to to the ATPL Skill Test. This can be done in the right seat and that should lift the restriction usually. And you are right, best is to give the CAA a call and to find out.
A check in the left seat would be another option, but usually needs additional training (upgrade training).