See: http://www.sheldonbrown.com/gloss_bo-z.html#bottom

British 1.370" X 24 TPI , right side uses left hand thread
French BB diameter 35 mm = 1.378" X 25.4 TPI . both side uses right hand threaded

French Shell is slightly larger than English BB but thread pitch is slightly finer.
Left side may seem to fit, but will be loose. If the Left (adjustable) side starts but then the threads bind, it is probably still a french thread shell. If the BB cup threads all the way without any binding, someone probably forced an 24TPI BB into the 25.4 TPI threads. The right side of french threading is opposite direction of british, threads shouldnt even start. It would be a total mess if someone were to have tried to re-thread the right side to british threadeing (completely changing the pitch and direction of the threads). If any re-threading was intentionally done by a competent mechanic, more likely that they re-threaded the french shell to Italian, this actually makes most sense because they are both all right hand threading, only the pitch and diameter change. Sheldon indicates however that the british threads are too small diameter to engauge in a larger diameter Italian shell (though pitch is same 24 TPI for both). .