There was a suggestion some time ago that with the aging of the VC10 of the Queen's Flight that an Airbus be commissioned for use by both HM and the Prime Minister. Given the travel schedules of heads of government these days, it would be Mr. Blair that would use any such plane most. Such an idea should be considered on its merits.
A minor royal has had many historical definitions. People think of the Duke of Kent or Princess Alexandra but increasingly it looks like all royals except the Queen's immediate family and that of the Prince of Wales.