Thank you all for the replies.
I don't think any FAA examiner will do the checkride in a G-reg, so I think the best way (best in the sense of having some flexibility on timing, without ending up not being able to fly IFR in the UK which is pretty essential for me) is to
Do the FAA PPL in a rented N-reg
Put the plane on the N register
Do the FAA IR in it
Incidentally
"You will not loose your IMC rating privileges on an N Reg aircraft. You may exercise all the privileges of a national licence in the country of issue in an N Reg aircraft"
The above is what the CAA told me, in writing. What they didn't know is that the FAA insists on any N-reg plane flying IFR having a pilot with an IR. So the IMCR does become legally worthless.