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.