If you are talking about just swapping the actual dial on your existing speedo, its probably physically possible, but I would have a doubt as to whether it would be accurate at all, as the movement of the needle may not correspond to the KPH scale unless it happens to be laid out in exactly the same layout as the small kph scale on your existing dials - which I doubt as most French cars have the speedo arranged to that the common speeds are laid out at prominent locations ie 90kph at the top.
You would be better off swapping the whole instrument pack over, but you would then be counting distance in kilometers too, and since it would no longer have a MPH scale, you would have the opposite problem when in UK.
The best solution is to just remember that 30 = 50, 45 = 70, 55 = 90 and 70 = 110. Its not rocket science.