All I'm worried about is, operating (potentially) across Europe, IFR in an N reg twin, with an FAA PPL (but no FAA IR or MEP), and EASA (UK issued) CPL\IR\MEP, based in France, am I legal?
I don't believe you are without an FAA IR/MEP as the FAA does not recognise EASA as a "country" for the purposes of FAR 61.3(a)(1). Had you retained your UK/JAA licence you would have been legal in the UK, but not elsewhere in Europe.
As I understand it, the CAA is advising UK licence holders to delay conversion to EASA licences until there is more clarity on FAR 61.3.
§ 61.3 Requirement for certificates,
ratings, and authorizations.
(a) Pilot certificate. No person may serve
as a required pilot flight crewmember of a civil aircraft of the United States,
unless that person—
(1) Has a pilot certificate or special purpose
pilot authorization issued under this part in that person's physical possession
or readily accessible in the aircraft when exercising the privileges of that
pilot certificate or authorization. However, when the aircraft is operated
within a foreign country, a pilot license issued by that country may be
used;