Do they not adopt the NL system, were the GenDec ist automatically generated when you file the UK-GAR (which you have to do anyways)?
The Netherlands tried similar a while ago and failed enforcement, btw.
And just to be over precise - you had to have a GenDec at any GA flight in all ICAO countries since the old 1949 days of ICAO Chicago, but todays interpretation is to have a blank form and fill it upon request of an official authority. I know, many ignore or want to forget, but the requirement for a GenDec ist stone age old.