I will have a go (for the fun of it). I have three licenses but I still get confused! IO540 will be along to correct me.
I hold a JAA PPL and from what I can understand, I can fly G-reg and N-reg in the UK.
Yes. Now the buts. You cant fly at night, unless you have a night rating, and you cant fly in VMC over an undercast unless you can see the ground.
If I want to fly the N-reg outside of the UK then I will need an FAA-PPL.
Maybe. There are two schools - you can and you cant. To be safe it is probably best avoided.
does anyone know what is required to convert my JAA-PPL to an FAA-PPL or obtain an FAA-PPL.
You need the paperwork from the CAA to say you are who you are and your logbook is what it is! You need to do the FAA theory - you get no exemptions for the JAA theory. You need to do the flight test, and will need to do as much additional training as prescribed by your instructor.
Can I fly an N-reg IFR in the UK/europe on a JAA-IR?
UK yes, Europe probably not, see above. However the foreign pilot conversion is a very straight forward route to an FAA IR.
Now, if I do have my FAA-IR (but not a JAA) then can I fly G-reg aircraft under IFR or only N-reg
Yes in the UK, but only in VMC and ourside CAS. You can extend your priviliges to IFR in IMC outside class A by applying for an IMCr which will be forthcoming without additional training. You cannot fly in class A in the UK or Europe whatever you do because there are subtle differences in the way airways physics work in Europe compared with the US which you can only learn by doing a JAA IR.
In short if you have a JAA PPL and think you will fly both G-reg and N-reg in the UK and Europe and want as much protection from whatever that lot at EASA cook up then you are best off doing a JAA IR, an FAA PPL and a foreign pilot conversion of your JAA IR. In that way you have the lot and can do just about anything time, money and other committments allow along as you always ultimately give way to the capabilities of your aircraft and the weather.