Basically, I don’t think you shouldn’t be driving a car using a foot that isn’t completely reliable. I used to find that my right foot used to slip off the accelerator, and at sometimes very inconvenient times too. Like when joining a lane of fast moving traffic. Or when on a motorway.
And I suspect that is what the DVLA would think too.
You can have an automatic car adapted so that both accelerator and brake are operated with the left foot. It might take a while to get used to, but you could do it. Or get your car adapted to hand controls, so not using your feet at all.
What I did some years ago, was to go to a mobility testing centre. They’ll test all your reaction times, your abilities to use your feet and make recommendations as to what sorts of adaptations should be used, if any. They usually have a test track and adapted vehicles for you to try out. They write a full report and will also recommend places to get suitable adaptations done, as well as driving instructors if you need to spend a hour or two practicing. It’s not cheap (about £140 when I did it in about 2007) and the adaptations aren’t cheap either - changing to hand controls at the same time cost about £750.
Eventually driving using a push/pull accelerator and a steering ball (to enable you to turn safely with one hand) becomes second nature. And I believe driving just with your left foot is equally natural after a while too.
Best of luck.