From a RAAF (1942) ground school training publication,
The air pressure in the induction pipe is known as boost or boost pressure, and is measured in lbs per square inch above (+) or below (-) that of the standard sea level atmosphere.
Of course, we colonials may not understand the complexities of machinery, but to me that says a boost gauge is exactly the same as a MP gauge. The boost gauge will indicate ambient pressure with the engine shut down.
Re boost cut out. The Lancastrian (transport Lancaster) engine lists a maximum of +14 boost, but with the boost cut out +18 was available. +18 was only to be used on take off if necessary, and if so used, boost was to be reduced to +14 as soon as the +14 take off safety speed (140 knots) was reached.