Well, it would appear he was in England by 1825:
https://www.familysearch.org/ark:/61903/1:1:NJQT-ZHSIt is quite hard to trace a family in France without a location, unless you get lucky - there is no.
http://www.rootschat.com/links/01li6/ - this could possibly be a reference? A Francois-Hubert Gassion, a resident of Caen (in Calvados), in 1822 sold some land at Castillon (presumably also the one in Calvados).
There also is another reference in the same book to a Francois-Hubert Gassion giving up title (I think?) of some land in Castillon to Marie-Anne Julienne, widow of Raphael Gassion, resident of Castillon.
http://archives.calvados.mnesys.fr/?id=recherche_thematique - a search of civil records here shows a birth record for a Francois Hubert Gassion
7 Messidor III (Revolutionary calendar) which corresponds to 25 June 1795.
To find this in the birth records you need the image set "1793-1812 Castillon > Naissances, Mariages, Décès Lacunes : N.M.D. (an IV-an VI). ", image 41 of 442 (I'm not sure I can direct link).
Extracting the pertinent: Raphael Gassion, resident of Castillon, a bricklayer, along with Hubert Gassion, day-labourer aged 39, and Marguerite Le Cordier widow of Francois Gassion, aged 60, both residing Castillion, declared that his wife, Marie Anne Le Bourgois, was delivered of a male child given the names Francois Hubert.
Various legal wording - Raphael signs R. Gassion quite neatly and Hubert (brother?) manages a more shaky signature.
This is not proof that this Francois Hubert = your Francois Hubert, but it is interesting that this man had a father Raphael, and yours gave his son the middle name Raphael, and that he seems to have possibly disposing of property in France in the early 1820s, soon before your man appears to marry in England. I think it would be worth investigating this Castillon family some more.