Most likely because he simply handed a paper flight plan to someone at his departure airport, and doesn't know if that will be excepted at his destination for a return flight.
1) Why can't that "someone" then handle the return flight plan as well ?
...will they accept it from him given that it's not his departure airport for the return leg.
Why should the flight plan be filed with the departure airport ? Surely UK briefers talk to French briefers ? (And vice versa : both legs could be filed through OLIVIA, no need to see your UK airport chap).
2) With cost efficiencies being implemented across the board (abolishing MET offices at regional airports and increasing charges to offset unavoidable costs), I'm amazed at the fact that a UK airport would still employ "someone who handles paper flight plans".
I'm not criticizing the thread initiator for trying to find out how to avoid wasting time on the ground in France. That is of course commendable.