The 'military' bit gets you off some or all of the mandated flying training. After some careful reading of CAP 804 I offer the following thoughts:
A FJ pilot can go straight into a SE CPL and then SE IR(A) skills test without any training, using the military credits. Then, he can do a ME class rating (6 hours' flying, first quote below) and become eligible to train for a ME IR(A) using the standard civilian 'crossover' rules (second quote, below). I wonder if an enlightened FTO could roll all this ME trg together into one 6-hour package?
The flight training course for a single-pilot multi-engine class or type rating shall include at least 2 hours and 30 minutes of dual flight instruction under normal conditions of multi-engine aeroplane operations, and not less than 3 hours 30 minutes of dual flight instruction in engine failure procedures and asymmetric flight techniques.
9 The holder of a single-engine IR(A) who also holds a multi-engine class or type rating wishing to obtain a multi-engine IR(A) for the first time shall complete a course at an ATO comprising at least 5 hours instruction in instrument flying in multi-engine aeroplanes, of which 3 hours may be in an FFS or FNPT II.
If a FJ pilot elects to go straight for a ME CPL and IR, none of the military exemptions apply and the standard civilian rules must be followed. For a start, this means that a PPL(A) must be obtained as a civvy cannot start a CPL course without one. So chalk up some puddlejumping practice and a PPL test. Now add a ME class rating to that PPL. Now do the standard civvy CPL training course (which will be expensive if you do it all in a ME aircraft) and the CPL skills test. Onto the ME IR:
8. A multi-engine IR(A) course shall comprise at least 55 hours instrument time under instruction, of which up to 25 hours may be instrument ground time in an FNPT I, or up to 40 hours in an FFS or FNPT II. A maximum of 10 hours of FNPT II or an FFS instrument ground time may be conducted in an FNPT I. The remaining instrument flight instruction shall include at least 15 hours in multi-engine aeroplanes.
10.1 The holder of a CPL(A) or of a Course Completion Certificate for the Basic Instrument Flight Module may have the total amount of training required in paragraphs 7 or 8 above reduced by 10 hours.
Ouch! I suggest that the latter path is pointless - you might as well grit your teeth and take a SE CPL and IR to make use of the military training credit. This unlocks a much quicker route to ME CPL and IR (6-12 hours, vs 60-70 hrs?).
The source reading for all this is CAP804 Section 4 Parts H and L (Appendices 3 and 6).