Arriving from EU (except from Eire and NI - see above) to UK/GB is legal if you submit a Flight Plan, and also a GAR (4 hours notice) to any airfield (including a farm strip).
Outbound Old Buck to Belgium is OK so long as you meet Belgian requirements (which are probably first Schengen landing at Customs field).
There's no need, travelling UK to EU (except Eire and NI), to advise anybody at the UK end of the journey.