Thomas C,
You aren't correct in saying that ALL UK military pilots are taught that air under the disc becomes compressed. At least it wasn't so in my time. I was taught that the ground cushion air behaved differently to "free" air, ie. air higher than 2/3rds rotor span. I don't ever recall it being referred to as compressed air, but more as a static or constrained dome of air beneath the aircraft which altered the induced flow through the rotor disc.
Perhaps you mean "military" in the army sense?