If you need to get cheap and relatively quickly to the airport it it worth to consider coach transfer. The first company I would like to recommend is National Express. Most of their coaches are fitted with comfortable leather seats, head rests and ample leg room. On some of the routes you can also enjoy free wi-fi. The coaches are clean and new. The standard of the journey is very good. They offer good amount of the daily coached to and from airports within whole UK. To check connections and buy ticket visit National Express website. They have often load thousands of fares for £5 into their system.
Another company operating but only London airports (Stansted, Luton, Heathrow and Gatwich) is Terravision. The also offer cheap transfers to and from airport. Please check Terravision website for actual rates and timetables. Please note Terravision operates also in other countries. Especially they are very strong in Italy (they are present on the most Italian airports), but you can also book airport transfer to/from airports like Madrid, Porto, Malta or Stockholm Skavska.
Very often the cheapest one to get from/to the London airports is easyBus. It is slightly less convenient due to smaller vehicles are operating. But often we can travel for very good price, if we of course book enough in advance. Please check easyBus website for their rates and details.