I just come up with this idea that the cheapest means to travel outside London zones into London centre, let's say Oxford Circus tube station. Start point must be a nation rail station outside any London zones including Watford Junction. Examples should be notified with weekdays, weekends; peak , off peak; single, return; with or without railcards, etc, but not all necessarily listed. Let me start an example: I start from Staines railway station weekdays off peak single without railcard: Bus 117 - Hounslow Central Tube - Oxford Circus Tube, total cost : £1.3 + £2.5 = £3.8 ,Same way return £7.6 but much time consuming than: weekdays off peak return without railcard: Bus 117 - Feltham rail - Vauxhall rail -Oxford Circus, total cost : £ 8 (London zone 1-6 travel card) This topic can also be extended to the most cost effective means travelling into London centre(Oxford Circus) from outside London zones. Please leave your suggestions, thank you.