The Prime Minister should have a dedicated aircraft.
If it is taken from the 9 operational aircraft this represents a loss in capability but if it from the 5 spare / leased aircraft it could represent an increase in capability - my money is on the former.
If the RAF is to get additional fast jet squadrons additional AAR assets could probably be justified but as an "armchair expert" the contract with Airtanker and the leasing airline may make this option unrealistic.