I use megabus to travel from Aberdeen to London - 13 gruelling hours!
It's not too bad, the best thing you should do is get right to the front of the top deck. It's much more comfortable if you can actually see where you're going and you have a bit more legroom. Also, take music! An mp3 is an absolute essential! I can manage 13 hours absolutely no problem if I have my (fully charge) mp3.
Don't bother using the toilet either. Wait till you get to the service station. Other than that it's cheap and gets you from A+B. It's just some of the passengers are a pain in the arse (fidgeting, talking and laughing loudly in the middle of the night when you're trying to sleep) and it gets a bit claustrophobic so like I said, get to the front, and travel during the day so you don't have to worry about trying to get some sleep.