Re: the UK use of an FAA PPL not a piggyback.
I have queried this before. In September 2017 I was in the UK and thought I'd do some flying, as Earls Colne is only a couple of miles trom jy UK house I went there.
Got an odd reaction from the guy who said their expert on FAA licences would be back in ten minutes. When the guy got there he told me, can't use FAA licences in the UK you have to do a full EASA course, so I just left - I think my mistake was mentioning that I lived in Barbados and owed my own airplane here - it left a nasty taste in the mouth.
I understand that things may change after the end of this year if one is still around! The good old fashioned two out of three rule worked well so UK airspace, UK reg airplane US licence is ok, fly to France and no two out of three - maybe that is coming back.