If a hypothetical Physics tutor at a hypothetical Oxford college were to be looking at the hypothetical application of a hypothetical student who has a hypothetical nine GCSEs at grade nine, and one hypothetical GCSE at grade five in French, and who has otherwise (hypothetically) presented a strong application, that hypothetical tutor might well (hypothetically) say "this hypothetical student had a hypothetical bad day at the office, but let's still interview him/her".
TL/DR? The presence of one five amidst a sea of nines would not be a hypothetical deal breaker.
The hypothetical French tutor at the very same hypothetical Oxford college might say (en manière hypothétique) "Vous plaisantez, n'est-ce pas ? Aller se faire cuire un œuf!"; but you would not be applying to study French, so who cares what that hypothetical croissant-botherer thinks?
But, as suggested above, why not bin French if you are not making progress? Or (no joke) go and live in deep rural France for a month - the best way to learn any language is by speaking it. Better still, go and live in North or West Africa for a month. African people in "La Francophonie" speak clearer and more grammatically correct French than many Parisians do.
I add to what a previous poster said (and this is a lesson for all of life) - worry about problems when they become real, or imminently real. Don't worry about problems that have not yet materialised, and might never materialise.
Bonne chance, et vive le sport!
Mademoiselle S Byng MA (Oxon), Avocate au Barreau de Londres.