Here's the issue using myself as an example (and a few people have tried to make this point). I'm possibly a HENRY (haven't checked - I might be counted as 'rich' under whatever logic is applied, but at minimum I'm a HENRY). I fundamentally agree in a progressive taxation system and the need to redistribute income/wealth via taxation. DH and I have big arguments over inheritance tax because I actually fundamentally agree with that as well because it's a means to try to level the playing field a bit on generational wealth as well.
However, I don't currently live in the UK. No plans to move back any time soon, but let's assume that I was going to. My options would be to work in London and either live in London or commute in from somewhere for (eg) 150k a year OR live way outside of commuter territory in one of the UK's second cities (eg Birmingham, Manchester, Leeds) and earn maybe 80k a year (guesswork but this is ballpark). The additional costs associated with living in London make living out of London the obvious choice for me - I will be far better off and I have no actual ties to London. But if lots of people like me make that choice, that's a real big issue for tax revenues.
The HENRYs aren't poor, and they're way better off than a lot of people, but they're also majorly important to tax revenues. All of the 'aww poor Henry he needs to budget better / move out of London' comments are missing the point is that as the country is currently set up we can't afford the Henry's to move out of London. We also can't afford for them to decide that they'd prefer to take a lower paid job with better work life balance/less stress/more social kudos (those are ors - not saying that you get all of those together ever!) than being an evil high income earner as society sometimes portrays HENRYs.
100k is nowhere near as much as it used to be, 50k also isn't. In the grand scheme of things Henry isn't a priority, and he shouldn't be. But if something isn't done about the attitude and the situation then more and more people in this bucket will think 'sod that' and stop pursuing these high income, London based jobs. These jobs are only in London because the industries believe the people want to be in London.
My view is that the focus is always on the wrong area - we need to be taxing wealth not just salary. No one likes that though because once you start to tax wealth, it impacts people on lower salaries who are sitting on expensive houses, and retirement accounts and investments. These people don't count themselves as rich because they don't think of the equity in their house as making them rich. They probably have a far greater net worth than Henry, and possibly than he ever realistically will have. It's easier to tell people like Henry off for moaning about not feeling rich on 100k than to accept that even if you feel like you're the squeezed middle your house equity actually makes you richer than Henry.
And at the bottom of all of this are the people that are really poor - they don't have the house equity and they don't have the salary. They have a right to be upset and angry at the cards they've been handed at life. But Henry isn't any worse than a pensioner sitting on 500k in housing equity who could downsize and not have to pay any tax at all on that gain, and in fact he's far more likely to have been a net contributor.