Coaches

Long-distance bus travel in England and the U.K.

With rare exceptions, an city-to-city coach is not going to be much cheaper (and certainly not faster) than the train or a no-frills airline.

The bus will only (a) take longer, and (b) be less comfortable.

These are not bonus factors when considering your options.

(By the way, this particularly goes for the London-Edinburgh route; the Caledonian Sleeper train is lovely; the overnight coach is hell on wheels.)

Coaches are really only useful for getting to places where the trains don't go, or when a train journey would require multiple tedious transfers.

Then again, these sorts of places—ones literally off the beaten (train) track—can make excellent destinations.

Coaches do have the benefit of sometimes being quite a bit cheaper, especially if you use Megabus, or have waited to the last minute (when train fares are highest). 

Oh, and a note for Americans: in British English, a "bus" operates within a city, a "coach" between towns. Use those terms to help avoid confusion in Europe as many Europeans learn a more British form of our lingo.