I can almost guarantee that Eurolines busses are the cheapest way.
discount airlines are good in theory and for saving time, but they usually fly to crap airports (especially in london) that are time consuming and difficult to get to. bus stations are generally much more centrally located.
example: I flew from Amsterdam Schiphol (5 euro each way on the train to the airport, 20 minutes each way) to London Luton (20 pounds return to downtown london, 1 hour each way). I had to get to the airport 2 hours early. My flights were each delayed 1/2 hour and took one hour each way. That's around 45$ just for airport transfers, and about 10 hours travel time all together .
If you book 30 days ahead on eurolines it's 39 euros for the entire trip. You can also take the bus overnight thereby saving money on accomodation. The bus takes about 10 hours each way. The bus is definitely not as glamourous or fun as flying though.
k, that's it.