I just did a post about Silverstone Racing Circuit via megabus and thought I might do a whole post dedicated to their great services. I've used them a lot and almost always get a better price than CityLink:

megabus.com provides low cost UK inter city travel with prices starting from as little as £1 + 50 pence booking fee!

Launched in 2004 megabus.com has since grown rapidly and now departs and arrives to over 50 destinations within the UK. The megabus network stretches from the Highlands of Scotland to south-west England, connecting some of the UK’s most popular destinations, including London, Birmingham, Manchester, Liverpool, Leeds, Edinburgh, Glasgow and Cardiff.

megabus.com launched with an online booking function meaning booking a seat can be done easily and quickly at low cost inter city travel . Here you are able to view all routes and destinations (megabus destinations), search and buy for the cheapest tickets available and purchase effortlessly. Passengers can also request to receive their unique booking reference by text message (a 10p charge is passed on to the recipient of the mobile). This saves on unnecessary paperwork and unnecessary environmental waste.

megabus.com has, since its launch, extended into rail and now offers passengers the opportunity to acquire low priced rail tickets by booking through megatrain.
megatrain travels to a number of destinations throughout the UK: From London as far north as Glasgow, London to destinations such as Sheffield, Nottingham and Peterborough and London to the South West, such as Portsmouth, Exeter and Bath.
megatrains are served by a number of operating companies of which further details can be found here megatrain

With a simple to use website and prices from as little as £1 + 50 pence booking fee megabus.com - low cost intercity travel in the UK is certainly worth a look.