There are many airbases in the UK, most are operated by the Royal Air Force (RAF), some have in the past, and a few still are operated by the United States Airforce (USAF), or are used jointly by NATO.

Although some are located inland, many of these bases are located close to the coastline, but most are still situated within or close to highly populated areas, which is OK for regular known aircraft to use, but not suitable for use by secretive aircraft.

Apart from that is, one built on the Mull of Kintyre 

What is Machrihanish?

Campbeltown Airport (IATA: CAL, ICAO: EGEC) is located 3 nautical miles (5.6 km) west of Campbeltown, near the tip of the Kintyre peninsula on the west coast of Scotland. Machrihanish is a few miles on the opposite coastline from Campbeltown on the Mull of Kintyre, where there is a designated docking port for nuclear and diesel submarines, and has the equipment and abilities to deal with nuclear emergencies onboard our submarines..

Machrihanish was once a working coal mine and has had an airbase in-situ since World War one. The base and airstrip are now operated by a private company (MACC) but still owned by the Ministry of Defence (MOD) and still under  'Care and Maintenance', meaning that although it is able to be used by civilians and as a commercial enterprise, the Military can instantly reclaim control during times of conflict or National Emergency.

The long runway at Machrihanish made it an ideal option as an Emergency Landing Site for the Space Shuttle.

Although the space shuttle never required to land on the runway, it is believed that many other aircraft have landed on the 10,003 ft (3,049m) strip, including  stealth aircraft such as the F-117 Nighthawk and B2 stealth bomber. It was designated as a covert landing site for the long range US recognisance aircraft the U2 and its replacement the SR71 Blackbird.

It is also believed that it has been used as a landing site for the newer top secret SR-72, Aurora and TR3B

Apart from having massive hangers above ground, which are supposedly used to provide camouflage to the top secret Aurora, and undercover refuelling for the conventionally fuelled secret jets keeping them out of sight from passing overhead satellites or the residents of Machrihanish, there is believed to be an extensive secret underground facility.

Built over an existing coal mine, this is more than speculation as the majority of work would have already been done. But there has been evidence over the years of large amounts of spoil nearby, and mining machinery that was too modern for the time the mines were in operation.

Nuclear weapons were stored at Machrihanish during the cold war, and these were believed to be stored underground, but the building of the underground facility was believed to have continued post cold war times. 

There is even speculation that an undersea entrance allows submarines to enter undetected from the bay, although there is no proof of this at this present time.