In terms of emissions, there is a big difference between buses (typically operating stop-start in an urban environment) and coaches (few stops, operating at a more efficient speed on longer journeys). Whether the bus or coach is at full capacity also makes a difference to the environmental efficiency. National Express estimate that its London to Birmingham coach, operating at full capacity, reduces emissions per passenger by 90% over the same journey in a private car.

Traveline offer regional and national coach travel information. They also list all bus stops on the mapping facility.

Stagecoach are a national and local bus and coach operator, although you may want to investigate their ethical ratings.

For European travel:

Eurolines

National Express

Eurobus Express