I used to skydive from a Ukrainian-registered aircraft based here in the UK (Let 410 as it happens). It had Ukrainian pilots and (I believe) engineer. It was here for years. The Let 410 type is unknown to the CAA, it was never offered for certification here. Nobody from the Ukrainian oversight, of course, ever turned up, nor from the CAA (in contrast to at my flying club on lighter aircraft). All the manuals and documentation were in Cyrillic anyway. So I guess foreign-registered aircraft based here are effectively unregulated.
One wonders in the Sala case how a chartering passenger would even know - the registration of the Malibu N264DB was applied such that the "N" bit was on the lower part of the passenger entrance, which hinges down to form the steps into the aircraft, so the passenger being taken by the pilot out to the prepared aircraft, even if informed about aircraft markings, would never even see where it was registered.