Traditionally at least, classes in the UK were based on social status, not the amount of money you have.

For example, I've known impoverished upper middle class people, people who self identified as working class (e.g. drinking in the miners welfare) even though they had millions in the bank and not particularly well off upper-class people.

https://en.wikipedia.org/wiki/Social_structure_of_the_United...

Edit: As might be expected, nothing about the class system in the UK is clear cut, consistent or particularly logical.