The answer is, as I understand it, that with any FAA pilot certificate while flying a G-reg in the UK, you are only allowed Private Pilot level, Day-VFR privileges. This is written in the CAA rule book somewhere. That's as much as I know, and I await to be corrected by someone who can quote chapter and verse or knows better.
I've never heard of a "combined PPL/CPL" test, surely you'd do one or the other? So simply add the Helicopter Rating to your FAA Commercial, or go another way and just get an FAA Private Heli Certificate based-on your JAA PPL(H). Obviously you'd need to meet the Private Pilot training requirements, as well as Commercial, but it wouldn't be 2 tests in one, it would just be the Commercial manoeuvres etc, and the boxes ticked for Private Pilot before undertaking the Commercial pratical.
I'm a CFI/CFII ASEL, so I'm not too sure about night endorsements (must be a heli thing), as with a standalone FAA airplane pilot certificate, there isn't a "night rating" or endorsement and it's just part and parcel of the training and certificate. You probably know this?
Also, as with any use of FAA pilot certificates in the UK... Just be thankful for what you've got! We're lucky to be part of the FAA system, and also should be thankful to the UK CAA (for once!) in that they allow FAA pilots to operate a G-reg, Day-VFR, with no need to sit any exams, pay them any money or do any flight tests. Come 2014, this will supposedly all go to sh1t anyway, so make the most of it while it lasts.
WHERE I DRAW THE LINE, and seemingly people like Bose get their underwear in a twist, is when people try and operate in the grey areas and knowingly bend the rules a bit, by inventing piecemeal ways of thinking they can operate to suit themselves. If you want to fly a G-reg helicopter in the UK at Night, then it's up to YOU to do whatever you need to, so as to be legal. You should know what is involved re: FAA requirements for certificates/ratings, or at least know where to look for them in the regs as you're already a Commercial Pilot, you just need to know just what you can and can't do in a G-reg heli on a foreign licence.
If you don't meet the requirements on your JAA licence what on earth makes you think you can override them by switching to another authority?
The answer is no.
I know what you're getting at, but...
On my JAA PPL, I don't have VP or retractable undercarriage differences training, and only an SEP rating. On my standalone (ie not based on a JAA licence) FAA Commercial, I have a complex endorsement, and a multi-engine rating. So that means I can fly a G-reg twin, day-VFR on my FAA certificate in the UK?! Or I can fly something like an Arrow as PIC/P1 on a G-reg with my FAA certificate, which I couldn't legally do on my JAA licence?! Seems that way (I haven't done either) as far as I can see. Same goes for Aerobatics. There isn't an FAA aerobatic rating, but I can't see anything that prohibits me flying aero's in a suitable G-reg on my FAA ticket.
Looking forward to somebody taking my post apart with suitable references to the relevant rules and regs!