Well for a start, each Aussie rating he has will require an APC (Assessment of Prior Competence) at an approved ATC training establishment at his expense before it can be used in the UK, then it's up to SRG what the MER will be.
A FISO licence will be issued (for a fee) IF you have a VALID ADV rating. FISO units are not necessarily busier in summer but as they're all VFR, given a clear sunny day in winter they can be very busy!(I know, work at one)