If the airway is Class A you can't enter it unless you have an Instrument Rating you can only transit the base at right angles to it
UK AIP ENR 1.1
4.1.6.1.1 Aircraft may, without ATC clearance, fly at right angles across the base of an en-route section of an Airway where the lower limit is defined as a Flight Level.
4.1.6.1.2 Powered aircraft may cross an Airway in VMC by day without compliance with the full IFR requirements in relation to the aircraft equipment provided that the pilot holds a valid Instrument Rating and that clearance is obtained from the appropriate ACC.
Of course you can cross under any conditions if you have both kit and IR and a clearance from whoever is the controlling authority. In the case described if you can't get to London control Exeter or Plymouth Mil might be able to get it for you.