I have done it - and WR's reply is along the right lines, but not quite correct.
You do have to apply to the Belgian authorities for permission in advance, but that permission is granted for a whole year. So no need to worry if you have to cancel - you can try again later, any time in that year. At the end of the year, you are supposed to write to the Belgian authorities again with details of all the flights you conducted during that year, for their information. I have to admit to having completely forgotten to do this, but they never bothered chasing me in the several years that have passed since.
The address I have is:
Ministere des Communications et de l’Infrastructure
Administration de l’Aeronautique
Direction Aviation Privee
Rue de la Fusee 90
1130 Bruxelles
BELGIUM
But this is a few years old, so check the PFA website for the latest address.
Schengen only comes into play if you departed from another Schengen country, so wouldn't apply if you departed from the UK. In any case, it is not connected to this authorisation at all - all Schengen means is that you wouldn't need to worry about customs, passport control, etc.
Feel free to post here or PM me if you want any more information.
FFF
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