Funfly
Advisory routes (as quoted by you) are not Class 'A' airways - they're Class 'F'. As such you can fly in them VFR without a clearance anyway, IFR if you don't want to participate in the ATC advisory service. Therefore it doesn't really matter what your altimeter is set on.
If an Airway (Class 'A') is notified with a base level of an altitude then the chances are it's associated with a TMA or it's adjacent to an airfield to allow Commercial aircraft to step descent into it. That unit will be providing separation based on
it's own QNH. That's what you should be using.
(Bear in mind 3000ft on the regional is always higher than 3000ft on an airfields QNH - I know which one I'd use to make sure I stay outside
)