As regards LICENSES:
In the UK, you can fly an N on a UK PPL or a FAA PPL.
Outside the UK, you need an FAA PPL.
There are bound to be additional concessions to the above (e.g. you may be able to fly an N reg plane in Zambia or whatever, on a Zambian PPL) but an FAA PPL is really a must to be sure. You can get a standalone FAA PPL, or a "piggyback" FAA PPL.
As regards RATINGS:
The IFR privileges of your IMCR are limited (by the ANO) to the UK and certain other UK territories. They are not limited as to aircraft reg but the FAA requires an N reg plane flown under IFR to have an "instrument rated" pilot, so, for the UK, validity of the IMCR in an N reg is a grey area.
The FAA IR gives you worldwide IFR privileges in an N reg and the FAA IR is why most people go N.
You can attach an FAA IR to a piggyback FAA PPL.