If as seems likely at present Apple do not make a version of Lion officially accessible to you as a Leopard user, you'll need Snow Leopard first. I know someone who just bought SL from PC World for £13. I don't know if this is general or if it was just a one-off at her local store.
Apparently you can upgrade to Lion from earlier versions of MacOSX but you need snow leopard to be able to access the app store. In theory you could download Lion on a friend's Mac provided you used your own apple ID and then copy it to a DVD as described elsewhere on this discussion board. You could then use this DVD to install Lion on your Mac running say Leopard.
On reflection, if you can get a copy of Snow Leopard for £13 that would seem to be the easier route.
Mick Burrell 