The UK has proven that the most cost effective way to recapitalize the disparate former Cold War tanker capability (think USAF KC-135 / KC-10 equivalent of RAF Tristar / VC-10) is through a partnership between the military, military reservists (US Air Force Reserve / US Air National Guard equivalent of RAF Full-Time Reservists + Sponsored Reserves who are airline captains and first officers), and civilian capital ownership of the aircraft fleet.

The most demanding roles - including National air defense Quick Reaction Alert - are now conducted by the Private Finance Initiative contractor owned Voyager fleet. The Voyager is an RAF-specific version of the Airbus A330 MRTT which any other contractor looking to expand can buy commercially right now (and the USAF could have had 11 years ago !).

For those that are interested in the wider picture as to how the UK ensures proper air defense, how tankers fit into QRA, and how well the civilian / military partnership works - here are a few videos:

UK QRA Procedures (overview/history then launch orders from 13:29):

https://www.youtube.com/watch?v=3hijzP1zzLI

QRA - AirTanker Perspective:

https://vimeo.com/139570722