Hi Moona,
I did the FAA IR and JAA CPL concurrently whilst at OBA. I ended up finishing the IR after the completion of the CPL so I did the full 25 hours. Doing the IR at the same time was in my opinion a worthwhile move. I got valuable instrument training which helped hugely on the CPL course - In addition if there was ever a backlog of students on the Arrow (as mentioned above) or for any other reason I could be getting on with something else.(There was never really a backlog with the Arrow whilst I was there but I flew twice usually, once with the Arrow in the morning for 2hrs and IR in the afternoon for 2 hrs)
To help with your question I understand one needs to do 25 hours training for a JAA CPL of which 15 must be visual flight instruction and 10 Instrument (5 of this 10 may be in a valid simulator). These 10 hours of instrument time may be excluded if you have a valid Instrument Rating or are waiting for it to be issued (PROVIDED THAT THE IR COURSE AND CPL CSE DID NOT OVERLAP). Now here may be the catch... I dont know if the 'valid Instrument Rating' includes an ICAO Instrument Rating such as an FAA IR or if it only pertains to a JAA IR. Check with OBA or even the CAA (sorry I cant help you much here!). One thing I will say though is that 15 hrs is quite a push at getting the cse done (of course it depends on your experience and ability) but I found the 25 hrs were all well used by myself and as I said I was doing the FAA IR concurrently so I had the advantage of 'extra training'
Good luck