I think technically you're right but legally it becomes a minefield and companies need to tread carefully. For example, if an airline would only allow vaccinated passengers but those under 50 have not yet been able to get a vaccine then it could amount to an indirect form of age discrimination.
However it's interesting to note the government are mooting this idea in their
COVID-19 Response (PDF, 1.6MB). They don't call it a vaccine passport. They call it a "COVID status certification", which could also include testing as well as vaccination status. It's mentioned in the context of opening up the wider society (paragraph 130) and international travel (paragraph 136).