But even though they are covered by the EASA regs my reading of this is that any aircraft - Part 21 or 'gap' - can be granted an exemption by the individual country's NAA, such as the DGAC, at their own discretion. Is that right?
@Peter Gristwood if the French really wanted to allow UK registered aircraft Part-21 aircraft (which are no longer EASA aircraft) flown by UK LAPL/NPPL holders, they probably could. Politically it seems unlikely at this time this would do so since it could be seen to undermine the EASA (and ICAO) rules.
On the other hand, the homebuild/historic 28 day agreement is long established, it is not particularly contentious and the fact that LAPL/NPPL holders can use it is more of a convenient side effect rather than any sort of result of the LAA lobby being stronger than those for certified aircraft.
There is no such thing as a 'gap' aircraft, they may be confusion around permits and their legal status, but whether an aircraft is Part-21 or non-Part-21 is defined by the Basic Regulation, as retained in UK law.