I think that @grapefruitsarenottheonlyfruit has it spot on. You have the poor: relying on benefits, the rich who have enough property and investments to be able to live without working. Then there's the middle which is the vast majority of people. And I then think class becomes cultural and is not income based but is how people choose to spend their spare money.
So you'd be looking at things like type of holiday, car and clothing and choice of leisure activity as well as type of area and house people live in.
It's very hard to define, almost impossible. And I think harder as time goes on as more people work in service industry so the traditional white collar work. So jobs and income don't reflect class.
I'd say lower middle is a 3 bed semi in a suburban area, probably has a caravan holiday in a park in the UK or a week's all inclusive somewhere like Majorca or Tenerife. In leisure time would go shopping, to the cinema and watch a lot of TV especially ITV.
The more middle middle tend to either camp or hire a cottage or villa here or overseas. Leisure time is more visiting museums, galleries and things like National trust. They may watch more documentaries and current affairs on TV.
I don't really know much about the upper middle class but I think that your children going to private/public school would be a major indicator of this group. I am also not sure what their interests and hobbies would be that differ from the middle class but probably more rural horsey pursuits.
It is not cut and dried though, and I am not sure at what point your own background effects your class once you are a fully blown adult. I am from a quite middle class background but my mum was lower middle and dad middle to upper middle.
I am now a uni educated single mum living on a new build housing estate in a terraced house working a lower middle class job. But still have a middle class accent and tastes due to my background.
It's complicated!