Sorry, but me thinks your figures don't compute!
For a new(ish) 172 fuel burn is, erm, slightly more than 15lph. You'd be better off looking at 35 as a better estimate - even then it would probably be more. Oil?
Check out how much maintenance you can actually do yourself. Your airplane would need to be on a public CofA and that limits what maintenance you can actually do yourself. What about engine replacement?
How much would your insurance be? For a new(ish) 172 you'd need a few £10's to cover it, I'm sure.
Nowhere do I see any allowance for annual costs for the maintenance.
Capital costs?
Landing fee's / parking charges ?
Office rental - you can't brief from the back of a car anymore!
Think long and hard, and I'd be doing a significantly more research. If it were that easy then everybody would be doing it!