For stations that aren't accessible, there already *is* a free taxi service to the nearest accessible service. That is how Access for All on the rails works everywhere
The "Access for all" programme is about upgrading station facilities; as that's infrastructure it all goes through Network Rail (though the money comes from the government).
The label for operational support is "Passenger Assist", which is a national scheme but implemented and publicised by
TOCs▸ individually.
GWR▸ describe theirs on their web site, and in the booklet
"Making rail Accessible". The relevant bit of text says:
We can also give you advice about the trains and stations you want to use, and how accessible they are.
If they’re not accessible for you, our Passenger Assist team will talk you through your journey options and find out what support you need. We aim to ensure that you can make as much of your journey by rail as possible.
However, for those parts of the journey where this is not possible, we will arrange alternative transport that is accessible to you, to the nearest or most convenient accessible station, free of charge.
They are rather vague about what that might involve, no doubt because needs vary so much case by case and they prefer not to create a more specific "right".