I'm slightly amused when you say AV will encourage tactical voting. At present I'm in labour heartland, so my vote is pretty irrelevant anyway, but say it wasn't- I'd have to choose between Liberal, Tory, Green (Not that they stood here last time), BNP or UKIP (Both of whom did). Now, only the liberals stood the slightest remote chance of beating labour (and it was pretty absurdly remote, even last election). Does that compell me to vote liberal, if I want rid of Kevin Jones? At present, yes it does.

AV would give me the option of saying, "Well, I _really_ don't want the BNP to win, so they can go last, and the UKIP just in front of them. My views are such that I'd like to vote Green, but after that Liberal Labour and Tory in that order". That neatly describes my view, and lets my vote be accurately taken into account. I dislike Kevin Jones, but I disliked the UKIP, Tory and BNP candidates even more.