Where in the world , apart from a communist state maybe, can you find your version of Utopia Nic?
The mega rich do pay tax to the IR but they have also found a way of not paying as much as they perhaps should. That is not illegal according to UK law.
The people who claim benefits and then do jobs for cash are breaking the law intentionally and knowingly and not only for the few pounds to put food on the table scenario, as you suggest.
I have no sympathy for the benefit frauds, I have every sympathy for genuine claimants that need the UK Social system to help them.
Can you categorically state that if you were in the position of being Mega Rich and you're paying tax and then your financial advisor tells you he can save you some of your hard earned money, that you would not take advantage of a legal loophole allowing you to do so.
The only way that this type of situation will ever be rectified is a flat rate tax for everyone, irrespective of how hard they work and even then some(socialist and communists) would moan that the rich should still pay more!