It was some years ago now but a member of our flying group, who was not british, obtained a visa to enter france temporarily. You may know that you must, from the UK, enter and leave france by one of the specified french airports having given the specified notice. If the Douane do not attend you may continue without taking any further action. In the case of the non european with us they had to report to the douane both inbound and outbound. This meant in both cases, the Douane did not attend in or out, a taxi ride to the local Gendarmerie to report. Other than that there were no problems.
The French embassy in London is the authority you should check with for best advice. The french system is forever changing during theses times.