I guess depending how the offer is processed at the tills there's a small chance you'll get the Heathrow Rewards points for the pre-discount amount, but I'd certainly not guarantee it. If you have a big appetite and expect to spend more than £20 you could always split your meal between places so have the first course in one place, and then finish off in another.

There doesn't seem to be any Heathrow Rewards promotions to stack with this.