WSO1
The fleet I am currently serving on uses the highest MSA figure, we do not make use of the sectors. Also of course an MSA (highest ground/obstacle +1,000ft) does not translate directly into a safety altitude if the ground/obstacle in question is above 3,000ft (If you were 20 nm out from Glasgow heading 120 and quoted 4000 as a safety altitude, based on the sector MSA, then you were wrong even if you are allowed to use sectors - it should have been 5000 in that case).
However, as to whether or not you are allowed to use MSA sectors depends on the rules in use by the Nav School at the moment. YOU SHOULD KNOW THESE, AND HAVE BEEN FOLLOWING THEM!! It is no good trying to bring previous knowledge into any RAF training system! They are nearly all inflexible. You learn the rules of that particular game and jump through the hoops put in front of you without questioning them!! If the Nav School rule is to use the highest MSA then you were wrong, period. It doesn't matter what other fleets, ATC etc do!!