So they say that they would pay more tax, but they don't? Hot air, sorry. Anyone can talk and campaign and vote labour. It doesn't actually mean they are doing anything practical.
Perhaps next time they raise it, you could mention it to them, as many people don't actually realise you can volunteer to pay more tax? Some of them might think it's a great idea.
I suggest you stop and actually consider what you just typed. I mean, I know it's a really common soundbite amongst Conservatives, but that's the problem; it's said between a bunch of people whose world-views it supports, and hence the fact that it's actually quite mesmerisingly stupid is never noticed. Think. Engage your brain. It's idiotic, that statement.
I assume you are aware that the government of the day determines spending priorities. This government have prioritised things such as tax cuts for higher earners, and implementing Universal Credit (at 16 billion and counting, and it's been horrendous for those affected) and spending a lot more on assessing disabled people for work than is saved by denying them benefits. I could go on. The current administration believe in rolling back the state as far as possible, and cutting state provision for the poorest wherever possible. So what on earth is the point in claiming that voluntarily donating tax would change that? You know as well as I do that it's utter bullshit. Paying tax voluntarily, without changing the administration, isn't going to achieve what those who do believe in those things want. It will just enable the government to spend more on tax cuts, and to fund yet more extravagant schemes facilitating ideologically-driven support-denial. How can you argue that this is more effective than working for an administration who would levy more tax, but spend it on their preferred priorities? 
To argue that people who are intentionally supporting a party who will levy higher taxes against them personally so they can help fund more for the worst off "aren't doing anything practical" is just silly. They are doing what they can to effect actual change, despite such change being against their own financial interests. And let's face it, if it were so impractical you would not vote Conservative, would you? And yet, something tells me that you do.
It's a whataboutery straw man aimed at undermining the positions of those who disagree with the greed-is-good, I'm alright Jack mentality. I mean, I get that attack is the best form of defence, but that one's shooting blanks.
But in fact, i think you've described exactly the sort of people I meant. Your friends aren't really talking about sacrificing 90% of their top whack, are they? They're talking about a few more % for them, whilst expecting the government to go after people like Philip green and the magic circle lawyers and bankers you refer to.
If their top whack were everything over a mill a year, I imagine they'd think 90% was okay. I mean, I appreciate the struggle is real, but I think they would just about keep the wolf from the door. As it is, they're talking about sacrificing income that may be discretionary, but would most certainly be noticed in a way I don't think a slight increase at the top levels would be. You notice a loss of income more, the less you have to start with. That's just common sense. And sure, most of the people I'm thinking about fall into the very highest tax bracket, but only just. To you, that's not high-earning. To most of the country, it's four or five times what they earn. And believe me, low earners work bloody hard for their wages, too. They just have to contend with the risk of threatened homelessness and the need to turn to food banks, on top.
And I'm sorry: just to clarify - you don't believe Philip Greene should pay more in tax than he currently does? Are you serious? Yes, of course he should pay tax at a sensible level. I can't believe anyone argues differently about a case as extreme as his.
Basic fact is, this is a moral issue. Is it right that some people are sleeping on streets, and families are in awful B&Bs, because in-work benefit levels don't cover rents, even for many working people now? Is it right that we have an NHS falling apart at the seams, and that we spend less than equivalently rich nations do on that service? And is it right that someone in the ranks of the 0.1% should be expected to contribute more from the very highest end of their salary, to change that, and to make the lives of the poorest less unstable, and to improve the health service for everyone? You clearly think not. I disagree. I think of people who have so little and are barely coping, and I think of the way we as a family are able to pay for private provision for our child. So many people I meet can't do that. I know some mothers who skip meals, because their children can't handle childcare either, and disability is expensive, and their partners are on minimum wage. The idea they could fund a private physio or art therapist or tutor on top is just laughable. We can, thank God. But it shouldn't be down to how rich a parent is, to meet such basic needs. Yet these days, it is.
I have to wonder: how many people in work, but on very low incomes, do you actually know? Because I am going to do you the favour of assuming that you don't. That their situations aren't real to you. I think your views might change, if you did.