There is the law. (Discrimination wrt a protected characteristic in this case disability) and
There is a public health pandemic. Which has killed 110k.
Your responsibility as a member of society is to do everything in your power to prevent the spread of the virus. It is the same responsibility that we all have.
Disability does not exempt you from that responsibility.
The Law on equality discrimination does not magically prevent the spread of a virus.
You need to do exactly what we are ALL doing in order to lessen the prevalence of this spread. That is - stay home except when not absolutely necessary. Just because you are in a 'bubble' doesn't mean you need to visit when not necessary.
You have to weigh up the consequences of the need for the visit - versus your unmasked journey adding to the spread. Then decide what is realistic. Could you catch Covid from another unmasked passenger and pass it to your mother ?
Personally I work from home, have food delivered. Stay home and go out without a mask, to exercise.
Would I like to visit my 83 year old parents ? Yes of course. Haven't seen them since October. Am I bored witless, desperate for company and lonely .? Yes. Like pretty much everyone else. I am ECV have had my vaccine .. still doesn't mean that if my disability meant I couldn't wear a mask - than my RIGHT still does not trump my RESPONSIBILITY not to endanger public health .
Your right under the law is not to be discriminated against. This is fulfilled in that public transport cannot and does not prohibit you travelling without a mask if you have a medical exemption. Being shouted at in an abusive way because you are disabled is a crime under the Criminal Justice Act and should be reported to the police.
It comes down to judgement. Do you need to be in a place where a mask is normally required. ? If yes than do it. But that need must be great enough for you to feel justified in risking other peoples health. (And your own - because other none mask wearers obviously make you even more susceptible) .