National Express advocates the introduction of dedicated peak-hour coach lanes on motorways and trunk roads to enable coach operators to provide much more attractive services to commuters, along with other ideas as part of its group vision (link below.)
http://www.travelwatchsouthwest.org/newslog090608.html

It's outside the "Travel Watch South West" area ... but aren't there already bus lanes on parts of the M4 and Heathrow spur?

The M4 bus lane is not for the benefit of buses - well not just buses.

It helps traffic flow and prevents queues.

Look it up - there is a webpage explaining how (something about a reduction in the number of lanes at one point).