Skype lite is simply like the google voice dialer - it dials your mobile number. So, whatever minutes you have on your mobile, they will get used. Also, any data required to initiate the connection (from the skype dialer) will be charged on your mobile.
I have an n900 with skype on it and an unlimited data plan. Call quality is not very good and sometimes they don't go through because of Skype's p2p design.
If you already have a Google Voice and Gizmo account, then you can use Google Voice to dial your numbers for free (in the US). I'm trying to use that as it goes through a single server (excluding whatever hops are necessary to get there) so it should be better in theory.
Since you're in the UK, you may be out of luck. Google is coming out with their phone in January supposedly so you might want to hold tight and see how the Gizmo acquisition/integration turns out.
In the meantime, Orange carries the n900 in the UK as was recently posted by engadget. The phone is nice, but not mainstream/hyped like an android phone or iPhone.
Also, Gizmo is currently locked down since it was acquired by Google. So, you cannot register a new account.