But Heathrow terminal 1,2,3 is not an underground station anymore than Victoria is an underground station because there is a tube station of the same name.
In the context of the rules I believed that Underground is taken to be a station on the London Underground system rather than a station that is underground for example the various Thameslink Stations and those being built for Crossrail.
The other critieria that needs to be met is that the station is part of the main BR network. It is a new station and so never was part of the BR system but surely it would not have been the intention to not allow new stations to be cited eg Conon Bridge?
An argument could be raised is that the station was 100% privately funded and that for period of time BAA have a monopoly over services to and from the airport stations and airport junction Such an agument is however blurred by the way Crossrail is being developed and of course it was not one put forward by you as a reason to not allow the station.
That said it is a game and rather than hold up progress I am content for you to offer an alternative to associate with PAD should you so wish.