I think there is the 1%, the ultra high net worth individuals, millionaires and the rest of us. The rest of us are a mixture of comfortable, just managing and poor. I would argue comfortable is the closest to middle class (in economic definition).
Those who can't afford to buy their homes (and are not renting by choice) by retirement age and are on low incomes as determined by Joseph Rowntree foundation, choose between eating and heating, can't afford food etc are poor. Ditto for those on minimum wage and below.
Those who can afford some normal luxuries (eating out in mid range restaurants regularly, holiday, odd cup of coffee) in addition to mortgage and bills and can afford to save for retirement, own a house/flat with the same number of bedrooms as the number of people in their home (or are just 1 bedroom short), don't need to watch every penny- just managing/average
Those who can afford nice car, multiple expensive holidays and private school, private healthcare, home that is deemed to be expensive in its location and most likely in premium location i.e. weybridge, Hampstead, Richmond, primrose hill, hale barns etc- above average good incomes, comfortable, high flying professional class, some inherited wealth.
Many people would fall somewhere between average and comfortable, and I guess may consider themselves comfortable even if they live in a flat/house in a regular suburb cos they have the nice car and the holidays and the private healthcare through work.
The millionaires- detached house in premium location, 6 figure salaries that start with 3-5 etc etc etc, more than 1 million in assets excluding primary home.
The wealthy- ultra high net worth individual-30 million in investable assets.
The top 1%- billionaires..