Megabus.com currently has a wintersale with 10,000 free seats available on English and Welsh routes for travel from 14/1/13 - 28/2/13. You only have to pay 50p booking fee per booking. If you book several travel at one time, you have to pay the booking fee just once.
Update: The Megabus UK Winter Sale is back!
If you don't manage to get a free seat this time there are still bargain seats available with prices starting from as little as £1 + 50p booking!