ASLEF District Line action

Unfortunately, the Earl's Court vote was short again:

eligible to vote: 128
voting papers returned: 68

yes: 53
no: 13
41.4% as a percentage of those eligible to vote.

You are not allowed to change the character of a listed building without appropriate consent. What constitutes a change in the character will depend on on the nature of the proposed works and the nature of the listing (what specifically is listed and why - for example the steps might be an integral part of the listing). Without knowing the details of either it is impossible to form an opinion about whether the ramp would be appropriate or not. Even they don't alter the character of the building, the council are correct to say that works cannot be done without prior approval - planning permission may also be needed.
Disability discrimination requires that "reasonable" adjustments be made - adjustments that require changes to the character of a listed building are not normally going to be reasonable.

You are completely missing the point, which is that sympathetic changes to listed buildings can take place but what is and is not sympathetic and appropriate development must be determined in advance. The considerations are not just aesthetic but also historical, cultural, structural, architectural and what is proportional among others - including the purpose of the change. Whether something is visible to the general public may or may not be relevant - it depends on the context you haven't given us.

While the specific case you are talking about may be an example of completely sensitive development (you haven't given anywhere near enough information for us to judge) the rule is a bright-line one designed for simplicity and to be able to catch all proposed changes of whatever intention, utility and quality. It's quite possible that had permission been sought in advance that it would have been given either as is or with only minor amendments or an alternative solution to the problem approved instead.

It is not about the rights of disabled people taking second place, it is about striking a balance between the needs of disabled people and preservation of our cultural heritage (and other considerations). This is why all the legislation talks about reasonable adjustments - which all but a very small minority of people who need the adjustments are in favour of. In the case of a listed building, it is reasonable to require advance consent for works that may have a significant impact on the character of the building.

Like almost all "what is the value of X compared Y" questions (including combination of "cultural heritage", "ancient woodland", "disabled access" (for any variety of "disabled"), "progress", "better connectivity", etc, there isn't a single answer (let along a single right one) as there are dozens (if not thousands) of details about the individual that need to be considered.