There are exemptions from the law on wearing a face covering on public transport in England for some disabled people (and other exemptions that might also apply).
In general, my advice on this one is:
- learn the exemptions in case you need to use them to deal with an overly officious person
- wear a face covering anyway
- if you can’t wear a face covering, don’t go to places where you’ll end up being physically close to people you don’t live with.
I’d treat face coverings like medical equipment that kids get to play with to see how it works before it’s used on them. Have them around, wear them at home, experiment with the different things available to find the most comfortable to wear. [The last time I checked things like plastic face shields counted as a face covering and might be better for some people than material over their mouths and noses]