Found it. It is not being provided to them for their exclusive use, it is an RAF refuelling plane that is being adapted so that it can be used to transport government ministers. Most of the time it will be going about its main use. It should save more than the cost of refitting over its lifetime of use (The US president has a plane for his exclusive use, Air Force One)
"An RAF Voyager A330 air-to-air refuelling aircraft will be refitted at a cost of about £10m."
"The government says the move will save about £775,000 a year as the plane will be cheaper than chartering flights."
"A source said the PM's flights, using either Royal Squadron planes or long haul charter, cost on average £6,700 per flying hour and the RAF aircraft would cost £2,000. It would be available for refuelling when it wasn't in use."
"The prime minister's spokesman said the plane's "primary use" would still be refuelling and it would continue to carry RAF livery, but 158 new seats would be fitted and it would provide "secure communications" for the prime minister."
"A government spokesperson said; "As part of the government's defence review, we have been looking at ways to make better use of the RAF fleet to transport senior ministers and consequently deliver savings for taxpayers.
"We have decided to adapt one of our existing Voyager aircraft so that, in addition to its primary air tanking role, it can transport Ministers and it will also be available for the Royal family to use."
www.bbc.co.uk/news/uk-politics-34864328
