Assuming you're a owner or part-owner, then you will need CAA permission (used to be DfT permission) to receive remunerated training/testing in a N-reg aircraft as per ANO Article 225. The CAA fee is £75 and the
form is here.
Yes, an examiner can conduct your MEP/IR renewal in a N-reg in the UK. There are a few dual-rated EASA/FAA instructors who are also EASA IR examiners such as myself. However, in the UK a EASA licence is rendered valid on a N-reg in any case via ANO Article 61A so you can use any EASA examiner, assuming they are aware of these privileges. CAA Flight Examiners Handbook, para 2.14 refers.