By far the cheapest way is to buy a LAA aircraft, then you can save huge amounts by doing the maintenance yourself, if you are mechanically minded. The absolute cheapest would be a Jodel D9 or Taylor Mono or such like and just 2-3 gallons per hour. Low insurance as no passenger and aircraft low at only £6-8,000. Proper aeroplane with 70-80kts cruise and better than a microlight, as an of those two will never depreciate in value.