OK, explanation I got back was - as some guessed - due to fractional ownership: (country, airfield and aircraft details amended for privacy)
Since 1st January 2021 the UK CAA have said that they don’t recognise our fractional ownership program of European-registered single turbines which we operate under private flying rules.
To adapt we are putting one of these on the G-reg and partnering with a UK operator to use their single turbine UK AOC.
Under a UK AOC this aircraft can no longer access this particular runway as it doesn’t have sufficient fire category (we now require CAT 3). It’s also a short, grass runway which under the AOC rules makes the use very prohibitive, for instance, when the grass is wet we couldn’t operate at all.
We have the occasional visitors here at Strathaven Airfield for the racecourse at Hamilton, including a Cessna Caravan. So was wondering if that would stop. But I think that aircraft is owned outright.