In other words, it's all relative.
Which is a different issue from the question you want people to answer - you're looking for an objective definition of 'rich' so that someone who is that rich can be taxed more.
The issue, of course, is that the more you earn then (typically) the more you spend - bigger car, better/more holidays, etc - so even if you're earning £100k you may not have enough left over to feel that you're rich. Although you are demonstrably richer than someone earning £50k.
The UK government/HMRC clearly think that £100k is the point at which you are richer than others, as this is when the tax allowance starts to taper off.
https://www.unbiased.co.uk/discover/pensions-retirement/managing-a-pension/what-is-the-60-tax-trap-and-how-can-you-legally-avoid-it
So, ironically, if you are in this situation, you probably do feel less rich.