The plane had a US registration and was being flown by Southern Air Consultancy? I wonder if that Southern Air is related to the Southern Air Transport that was owned by the CIA. Involvement in the company by the CIA might explain the US registration.
Nah, no conspiracy theories needed...:^)
Lot of UK pilots get their licenses in the US (Florida especially - kids and wife to Disney World, dad to the airport) and fly in the UK on that FAA license.
The same is sort of true for having N-registered (aka US ones) in the UK. The maintenance is easier and instrument flying is easier with that FL issued license.
In particular the US Instrument Rating is easier to get and keep over the UK equivalent I gather. This link talks about it a bit - https://www.caa.co.uk/Commercial-industry/Pilot-licences/ICAO-third-country-licences/Recognition-of-ICAO-third-country-licences-in-UK-airspace/ - but of course the EU weenies regard it as a loophole another reason for Brexit...:^)