SAS UK's HQ is not as large, but it's pretty grand. Take a look at this PDF to see some nice pictures and to understand the history of the Thames-side, 110 acre estate, located west of London. Originally owned by Lord Devonport who made his fortune from tea, the house features its own ghost: the "grey lady".
I'm told that part of the deal to buy the adjacent land in 1997 was a promise to retain the cricket pitch and its availability for the local village cricket team. True to their word, SAS have restored the pitch, built a new pavilion, and made all of this available to SAS staff, the local cricket team, and the local Scout group.
In SAS's care since 1985, a visit to Wittington house is always a pleasure.