When you leave airspace that is using an aerodrome QNH to fly on an adjacent airway, the mimimum cruising level on the airway will be published as a Flight Level, therefore you must change the pressure setting to standard and adjust to the relevant FL as per your clearance.
The minimum usable FL on the airway is determined by a table taking into account the Regional QNH, this ensures terrain separation.
Controllers will issue an appropriate level, just be aware when the pressure is low, your planned FL may not be available