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I'd say "a British surname of French origin", or "a French surname of British origin", depending on the direction in which the name travelled.
For example, Newmarch is a genuine English surname, but it derives from the French place name, Neufmarché.
D'Oillenson is French not English, but it started out as the English name Williamson.
On the other hand Latham is an English surname that is found unchanged in France, where it has existed since the 15th century. I'd call that an English name with a French presence.