All they need to do is pass the MEP Skill test and written examination at an approved FTO.
LASORS Section F 9.1
Applicants wishing to gain a MEP Class Rating who
hold a valid non-JAA MEP Class Rating but who do not
meet the experience requirements in items i) or ii)
above will be required to attend an approved FTO and
complete training at the discretion of the Head of
Training. Applicants will be required to pass a written
theoretical knowledge examination and a Skill Test as
well as meeting the experience requirement of item iv)
and the requirements of item v) if applicable.