I can understand it’s upsetting, if not asked before…but I don’t think this would be seen as discrimination, it’s really for the benefit of people requiring adjustments.
A blue badge is essentially just an accessible parking permit. For wider facilities, supporting evidence can be requested, to ensure the correct facilities are allocated to the people who need them and the legally-required reasonable adjustments can be made.
At a quick Google of other festivals’ accessibility arrangements…it looks like Download and the BBC Radio Two in the Park asks for free registration with a scheme called Nimbus Access Card, which would require similar supporting evidence, Green Man asks for PIP, DLA or GP evidence for certain accessible arrangements.
I'm not exactly sure what you mean by 'for the benefit of people requiring adjustments '. As far as I have seen the only adjustments given on accessible fields are that you are closer to the arena and accessible toilets and showers and in the case of Cropredy close to the scooter charging points.
People over state pension age are not able to get PIP but can get a blue badge with disabilities, physical or hidden and it is not an easy process to go through. I don't want to get into a discussion over what can be seen as a disability, but I don't think it should be down to the ticket sellers to make that decision.
