The difference between some of the examples you’ve mentioned and a theme park is that in the theme park case, there is the option to do something else if a certain ride breaks down. You don’t go to a theme park to go on one particular ride (in most cases), you go for the whole experience and a variety of rides.
In the case of the flight, for example, you take it to go to one place, and if they only take you halfway there, they blatantly aren’t providing you with the advertised service. Whereas at Alton Towers, you aren’t going there for one specific ride, you’re going for a variety of rides and the whole experience.
Rather than it being like you going to a restaurant and them only bringing half your order (as you argue), I’d argue it’s more akin to if you went to a restaurant and they weren’t serving a particular dish due to them having run out (which I have known happen plenty of times and have every sympathy with). They are serving a reduced menu, but there are still plenty of other things to pick from, and you can still have the fundamental service you paid for.
Alton Towers state in their T&Cs that they reserve the full right to withhold any and all attractions from the product as they see fit. The park does not sell themselves on one particular attraction or set of attractions, they sell themselves on an overall package.