Check in at Paddington for the Heathrow Express stopped shortly after 7/7 for security reasons IIRC. Of course, many, many years previously, in the 60s, BOAC, Pan Am and various other airlines offered in-town check in, generally around Victoria, and then a bus to London Airport as it was then called. In the 80s, there was in-station check in at Victoria for select airlines using LGW.