I would think it was more likely to be chambered in 11 MM French Ordnance caliber The french round is about .44 caliber. I don't think a French or Belgium revolver would be chambered in a British round. The British did contract several Spanish gunmakers to make revolvers WW I, but no Belgium ones. There is no reason it can't be chambered in .455 or .455/476, in my opinion it doesn't seem likely. If it was exported to England and a British officer purchased it for his own use, it would be covered with British markings. JMHO and I have others just as humble.
As hrf posted, pictures would be great. It could also be chambered in 11MM German, the Germans seem to like invading Belgium and the other low countries.