Not a requirement, any more than it is for getting on a train. I am surprised you haven't seen this before! Been like this since - well, apart from one brief period after 9/11 - at least 4 decades.

BA do ask for ID if checking in at the counter (typically for checked luggage), but HBO needs no passport or other ID and I don't travel with one in that situation. If asked for ID, your BAEC card will normally suffice, though the police - in certain circumstances - are able to ask for ID too and probably won't be so keen on the old shiny one. Other airlines do ask for photo id, but that is for revenue protection.

See also:
http://www.flyertalk.com/forum/briti...-domestic.html

http://www.flyertalk.com/forum/briti...c-flights.html

http://www.flyertalk.com/forum/briti...al-flight.html

http://www.flyertalk.com/forum/briti...ic-flight.html

http://www.flyertalk.com/forum/briti...-photo-id.html

http://www.flyertalk.com/forum/briti...-domestic.html