I know that posters on JB will be quick to rubbish such an idea - they did when it was suggested for Tony Blair - but all the worlds principal nations, and few few too many banana republics have an aircraft for the use of the heads of government / state and it really is about time the UK had the same.
Having the PM or Monarch turning up at major summits or state visits in some piece of kit chartered on the open market doesn't give the right impression of UK abroad - unless we want to give the impression of a cash strapped, insignificant island that can't afford anything better!