I consider myself class-indifferent so I don't know where I'd stand in this.
The British are so obsessed by class. It's very sad
Middle class children should still need to work to achieve their goals when they grow up. Those who don't need to work whatsoever are obviously upper class.
There isn't a clear cut anywhere, and it's definitely not just about the money people earn and have. It also supposedly represents a culture and social rank.
Everybody identifies themselves there because in the middle you are less likely to be discriminated against, and you can easily victimise yourself for not having welfare whilst still not being rich enough to not care about having them.
I think one big difference, however, would be that if you are upper, your parents' money are likely to come from inheritance, not necessarily hard work; but middle class work their way up there. This would be why Liz II would be considered upper whilst JK could still be considered middle.
Middle class isn't a big deal. Society has made out like being middle class is amazing. It just means you're parents don't work in a pie factory in the north of England.