Megabus are doing their free winter ticket offer (again) for travel Monday to Thursday during the period 12 January to 12 February. There is a booking fee of 50p but if you book more than one journey or for more than one person on the same transaction the fee is still only 50p.
Details can be found at:
http://uk.megabus.com/free-seat-sale.aspx

NOTE Tickets are usually relased in batches so if you don't find what you are looking for its worth waiting a few days and trying again.