Gates and Buffet are arguably spending their billions far more effectively than the UK government, who are frittering our taxes away on trivia like libraries, cycle lanes, IVF, teaching assistants, eczema treatment, road sweepers, fire brigades and a standing army. Gates (and Buffet who's essentially given control of his dosh over to Mr &Mrs Gates) are spending the money where they can get the absolute most utilitarian bangs per cent in saving lives. From a strict utilitarian perspective I should probably be helping them overthrow the UK state and impound the Treasury.
Which is not to say that there aren't some other billionaires out there who totally need taking out. Unfortunately they tend to be quite heavily protected by large men with machine guns, and in some cases actual armies. I'll be cheering you on from a distance.