A long time ago this was possible. When they closed off pedestrian access from the north side of the airport (T1-3) they offset that by making trips on local buses free to/from anywhere within a mile or so of the airport. I'd plan my walk to end up at the closest bus stop on Bath Road (just east of Sipson Way, IIRC, but any local can tell you), then accept reality and let a big red thing with wheels and an engine take you from there on in.