IO540,
When flying to France from the UK, you do indeed need to make your first landing at a Customs airport, so the requirement to call an ATSU to close your flight plan is unlikely to be relevant.
But if I understand the French rules correctly (I'm sure CF will correct me if I'm wrong), if you were arriving from another Schengen country, there would be no requirement for customs - but a flight plan would be required nonetheless.
This aside, you can file a flight plan for any flight you like, whether it crosses the national border or not. There are some pretty mountainous areas in France where I would imagine filing a flight plan would be fairly normal, and ATSUs are pretty hard to find?
FFF
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