Up until 2008 Virgin used to provide an excellent non stop coach service from Watford Jnct to Heathrow and back at a very reasonable price. I would catch a train from Birmingham New Street and arrive at LHR within 2 hours.
Since then a stopping bus service has put pay to that! It's about a 3 hour connection time now. National Express provide a coach service from LHR to BHX airport, but it usually goes around the houses and takes up to 3 hours, so there is no real incentive for LHR passengers fly from BHX these days, or reverse. Nowadays the cheaper option is to fly via AMS, FRA or CDG for long haul flights so the pain of APD is less, rather than commute to LHR and pay full whack!