Don't go using the GPS for determining distance below LSALT as it needs to be IFR compliant.
AIP GEN 1.5 8.5.4.1 b
Night VFR RNAV. Pilots operating under the VFR at night may use GPS for:
(3)
deriving distance information for en route navigation, traffic information and ATC seperation.
The flight crew have to hold appropriate qualifications and the GPS receiver must meet the appropriate TSO standard.
Unless you use the published information, LSALT becomes a function position using the GPS anyway...
AIP GEN 3.3 4.11
An aircraft must not be flown at night under the VFR,
lower than the published lowest safe altitude...
I suppose it all comes down to whether you consider the MSA as being an indicator of LSALT as it is published on an aeronautical chart.
I agree that it is legal, but the regulations are very subjective!