You would need to have a valid multi-pilot TR included in your licence and meet the flying experience requirements for the ATPL(A).
Most people would do this in their first year of people-tube driving, but since you only really need an ATPL(A) to be
in command of an airliner, there's no great rush to 'unfreeze' a CPL/IR with ATPL knowledge - no-one who hasn't already qualified for an ATPL(A) from RAF service is going to go straight from the RAF into the left hand seat of an airliner these days. Most proper airline companies also operate strict seniority rules.....and are interested in 'relevant experience commensurate with age' when hiring newbies.
Whether the 500+ hours mult-pilot time already accumulated in the RAF will count, I don't know. It may well be that these hours have to be gained on 'civil airliners' (in simple terms) included in your CPL.
Do I detect that an increasing number of people are keen to bang out of the RAF?
Remember that there is a pretty large number of desperate youngsters out there who will even pay for their own type rating just to try to get an edge these days.....and who then find themselves being treated abysmally by some bottom-feeding lo-co airline which pays peanuts.....
