No idea. Presumably they can't just lease a military one (in an Air Tanker A330-type arrangement), and I'd be surprised if an air force anywhere would sell one to them. Even if they did, they'd presumably have to try and get it certified for civilian use.
Their Chairman is Sir George Zambellas, First Sea Lord and Head of the Royal Navy from 2013 to 2016, so presumably he at least has asked that question and got a satisfactory answer.