From what you say, it sounds like you’ve already got a French licence (or have I misunderstood…??)
If that is correct… I think you might need to take the HGV exam in France to get it on your French licence…
Did you actually take the HGV exam in UK ??
this link explains that a straight transfer is no longer possible in some situations, when applying for a French DL.
But the link also gives useful info and yet more links to the FB page …