Out in the dark
Where does it say that an FAA PPL who holds a qualification to fly at night cannot so so in a UK registered aeroplane? Applying for a UK PPL Night Q can be a problem if the applicant hasn't got three hours dual logged as part of their night training.
IFR is not a problem but maybe IMC could be but depending on the FAA qualifications held.
With regard to an FAA PPL flying a UK a/c outside of the UK, each state to be flown into so decides. Just need to ask each state unless provision is already in place.