I must have missed the election. When did someone get appointed to speak for the "entire disabled community in the UK"? Was it you who was elected?
Whilst it would be lovely if places could cater for all disabilities and the combinations thereof for every single person, then it is impossible for somewhere to be accessible to every living soul on the planet - and that includes many people who would be classed as "able-bodied". And yes, many places could make more effort. I am the first person to call out carelessness and discrimination in access terms. But I go to many venues for all sorts of things, and the vast majority of them are not impossible and try hard, and many listen to feedback positively (when delivered calmly and rationally - not demanding that people "educate themselves" and being provocative and insulting in making over the top claims that are clearly not true).
The only way you can book is over the phone. Same with theatre tickets, most cinemas, festivals, attractions. Most of them have little to no accessibility information online.
I literally cannot recall the last time I found little or no accessibility information on line for any of the events / activities that I have gone to, and seldom have to phone unless I need to check something. So this may be your experience, but it isn't mine, it isn't that of the "entire disabled community in the UK", but it will be shared by some people. You are assuming that all disabled people are the same, share the same experiences and the same concerns and complaints - that is incredibly arrogant and totally naive. We are as different from each other as any other group of people. We have some shared experiences, but we are not the same.
On the specific issue here on this thread, however, I would be interested to know what you think Butlins are lacking. You can find access information on the site, and you can book ground floor rooms by (stating the bleeding obvious) going to be section that says "choose a room" and selecting "ground floor". If you don't want to do that or can't do that, there is a telephone number to call, prominantly displayed in multiple places. If you also can't do that, you could go to a travel agent. That's options that don't involve the other obvious answer, which is "ask a friend". What are Butlins supposed to do - add smoke signals? Set up a prayer line? Physic paper?
@SurferRona who has clearly not read most of the thread "All the younger PPs here saying ‘well, site looks ok for me, so you parents are clearly idiots’ won’t begin to have any insight into your parents world... As technology progresses I increasingly see it just leaves more and more older people behind, it’s exclusionary, othering and discriminatory".
What an exclusionary, othering, discriminatory and arrogant assumption. Which you would know had you read the thread. Speaking as a 67 year old disabled woman I have not been left behind by tecnology and can manage to book holidays and activities etc - even my shopping - online with ease. And my father, who departed this world years ago but would have been over 100 years now was online! A minority of people over 65 are not online or may not fully understand it. That is why there are still phones and travel agents etc. Technology may be a barrier for a minority, but it is a liberator for many more. I have plenty of "insight" into the OP's parents world (as does my tech savvy 89 year old neighbour) and it is very simple - they made a mistake. They didn't read carefully the very prominantly displayed access information, and assumed that a company would know that they were disabled amongst the many thousands of bookings that are made. Everyone can make mistakes. Everyone does make mistakes. Butlins didn't make this one, and they in no way contibuted to it.
You would do better to "stop, and think" and stop speaking for us - we are not stupid (as you rightly say) and don't need your empathy because there is nothing wrong with us. Mistakes happen. The problem here is the people, including the OP, playing the "card" instead of accepting that they made a mistake. This has nothing to do with disability or age and everything to do with not reading the available information and making assumptions.