Howard wrote:
would it have been possible for you to have closed the flight plan after you had landed in France by calling the AFPEx helpdesk in the UK
Anyone with AFTN access can send an arrival message. The question is if they do. Since they need to know the departure and destination aerodromes, and departure and destination times, they might want to do it only if they have the original flight plan. So for a flight not involving UK airspace, they might not want to do it.
In france, you could simply call a BRIA (numbers are in the AIP and VFR atlas).
For flightplans filed via autorouter, you can also do it via the telegram interface.