RichmondPark1 Most billionaires acquired their money by building up big businesses and much of their wealth is based on the value of the business. To give all the money away they would have to sell the company to someone else. Not an easy thing to do if you own a business valued at £1 billion or more
Others earn their income by investing in other businesses, and again, would need to sell up all their their shares. Where they hold very large tranches of shares this could destabilise the company, and again it may be difficult to find someone in a position to buy a million shares in a company. In other cases the money is invested in land, which again would have to be sold.
Someone with a £1 billions of assets, will not have an income anywhere near that amount, only what those assets earn, which will be a lot, but nowhere near £1 billion.
Many very wealthy people have set up charitable trusts, and given most of their wealth to the charity. It is what Bill and Melinda Gates, have done and also others, like, I think, George Soros, and Warren Buffet.
Many more wealthy people give quietly. The singer George Michael was not a billionaire, but after he died it was revealed that he was a very generous donor to charities of all kinds. Nothing of this was known when he was alive.
I must say, if I was wealthy enough to be able to give enormous sums to charity, I would keep it very quiet indeed. Otherwise, on one side people would be fawning over you and saying how wonderful you are and how generous, on the other side, you would be inundated with begging letters. Either reaction would make me shudder.