Are you sure?
I always bought a coachcard for £5 every year (you need your crew ID to buy it) and then use that when I booked online or at the counter. It still gives the coachcard option now on the route I do LTN-LHR, so I don't see why you wouldn't get a discount LGW-LHR.
This is me after 10 years of commuting on the National Express.
YouTube - The Divine Comedy - National Express clip