It's been a while since I've paid attention, but the on-board announcements do (or did) very clearly say something along the lines of "The service is free between Heathrow Central and Terminal 4 [now Terminal 5 I guess] IN STANDARD CLASS ONLY" during the spiel about how the train operates.

When I have been in First on the HEX it's actually been rather annoying to be invaded at the end there by people who'll gain at most a couple of seconds of time once the doors open? Not when I barge in front of you, you won't. Some even chance it as soon as the ticket inspector has been around. The sheer audacity of people amazes me. I've seen such oiks ejected by inspectors ^ I don't expect someone from ET to plonk themselves next to me in 1C so they can be first out the door at LHR and I'm sure you don't either. So why do this on the HEX? I stick to the class I've paid for. That includes positioning myself at the doors.