Why is the "Roman" part at the beginning of "Romano-British" but at the end of "Gallo-Roman"? Why not "Romano-Gaul" or "Britano-Roman"?
Is it because the Roman Empire controlled Gaul for longer than they controlled Britain, so the British were not as Romanized?
I believe it to be as simple as the difference between French and English grammar.
In French, adjectives typically follow nouns, and the French term for 'Roman Gaul' is Gaule romaine. Hence, Anglicized and turned into an adjective, Gallo-Roman.
In English, adjectives typically come first, hence 'Roman Britain' = Roman Britain, or Romano-British.
The difference is not referring to degrees of Romanness, it's just a peculiarity of language.