Another thing that has been missed is that if you start the trip from your home base and land out of the UK you are burning duty free fuel for the whole of the flight, land some place on the south coast and that is were the duty free fuel starts.
Don't forget to file the HO60 form to HMRC !
Also try to clear French customs & immigration at an inland Airfield, LFAT is a great place to go but the landing fees are high for France. Le Mans, Tour & Troyes are my favourite ports of entry into France, all very efficient and inexpensive.