1939 and 1947 airfield plans are available from Hendon. These show the building drawing numbers...using those with references such as the drawings and standard texts such as British Airfield Architecture, British Airfield Buildings (volume 2 covers Expansion era) and Works will give you a lot of information about designsYeah I heard there was an entrance in the guard house but I couldn't see anything! Like you said more than likely blocked off now.... I wish I could find the blueprints
Haven't been to Upwood for around 20 years but there are no tunnels under the Guard House & Fire Party building.
As mentioned above, some of the Barracks blocks had standard air-raid shelters