Hi David,
The Alternative Vote System is not a proportional system, rather it is a preferential system. It is perhaps indicative of this that the main proponents of AVS are Liberal Democrats or Labour supporters because they would list the other party as their second preference.
This would be totally undemocratic in the UK and result in a skewing of vote outcomes to favour the larger Labour party – the Conservatives and any other party would be hopelessly outnumbered.
Personally, I would advocate a PR system, such as Mixed Member Proportional whereby national voting outcomes are reflected in the makeup of the national parliament.
For more information see an interesting blog by Simon Kaye on ATV at: