Many French pedals with BSC (British) threads are labelled "D" and "G" instead of "R" and "L" - example are Look S2 mtb pedals.
If you have a pedal you know is British, hold the spindles of that pedal and hte mystery one together (both right or left pedals, not right/left...) and see whether the threads mesh.
When trying to screw a British pedal into a French crank, it'll go in fine for about 1/3 of the way, then get really tight.
A French pedal in a British crank will be loose.
Any good bike shop will have taps to rethread French cranks to British. It is easy, but as with any threading, done wrong will mess up your crankarm.
Jan Heine, Seattle