French being 35 mm diameter is a little over 0.007" larger than British, which is about the thickness of two sheets of 20 lb paper. Not much, especially considering the loose tolerances of many European manufacturers. The 1.0 mm pitch works out to be 25.4 threads per inch, vs. 24 TPI, so what you have when you put a British cup in a French or Swiss BB shell is a cup that starts to thread in easily (perhaps even loosely), but gradually tightens up as the thread pitch mismatch comes into play. Bicycles being what they are, it should work out just fine the vast majority of times. I would warn a customer before doing it, but as a way to get an inexpensive bike back on the road cheaply and quickly, I would also advise they go for it.