Im not being funny but there always seems to be a comment like "Join the RAF" and get flying for free. You make it sound like you just turn up at Cranwell and you'll be on the Grob-Tucano-Hawk-Tornado route without breaking a sweat.
Intial advice is to get to Gatwick with £300 in your pocket to make sure there is nothing inherently wrong with you. This has to be the very first investment.
Second investment is the GAPAN test to see if you have what it takes. At £100 or so, its worthwhile.
It will take a few weeks to get the above two sorted out. Which gives you ample time to go through PPrune Stickies and archives and you will know absolutely everything about the routes to becoming a pilot.
Then come back and ask specifics for clarification.
In short you have asked for the cheapest route. Cheapest would be to apply for Cathay who seem to be recruiting like they will face a shortfall of 300 pilots by 2011. November sees the second round of London interviews, just 2 months after the first. You are too late for that by the way but could be lucky for next year if you apply now.
I would add though that you should at least enrol on a PPL course asap to show that you have started the process of achieving your dream. Eitherway, you will be going via the mod-route which is the cheapest. You will find hundreds of threads on this susbject.