Unfortunately, you do not say what types or models of "old British bikes" they are? So makes suggesting anything rather difficult??
However most riders used to left foot gear change do seem to be able to get used to right foot gears and left foot brake with a bit of practice, being able to mentally 'switch over' according to which particular bike they are riding.
Have you tried the right foot change? As you may find that it is not as difficult as you think?
And I suggest that you give it a try before going to a lot of trouble trying to convert your bikes.
