As new stock comes in access to the trains is less of an issue because accessibility is built in more effectively but attitudes of passengers who have made themselves comfortable in the areas designated for wheelchairs or individuals with mobility impairments are another barrier.  On board staff aren't always willing to tell people to move or to intervene if they are harrassing disabled passengers.  I've heard many horror stories.

Differences in platform heights and curved platforms means that the gap between train and platform various hugely.

As you point out, getting to the platform is the major issue.  Old infrastructure means that many railway buildings were built  donkey's years before disabled people started demanding independence.  Lifts may be inadequate or often not working.  Training for station staff is crucial when it comes to supporting individuals on and off trains and through stations.  One conference delegate stated to Arriva trains that he was fed up with staff refering to him as 'the wheelchair' over the tannoy when calling for the ramp to be brought out to get him onto trains.