Here's a quick link to a BBC article that shows how 2005's election outcome would change under various voting systems. One of the issues for the UK to consider is how difficult it is to implement these different systems. For example, moving to Alternative Vote (AV) where you simply rank candidates could be done very easily - no changes in constituency boundaries or in the number of MPs per constituency. However, if we moved to Single Transferable Vote (STV), we would need to design larger, multi-member constituencies which would take quite some time to sort out I imagine! Of course, the ones take a little longer to implement also seem to be the ones that are more proportional. More on this later!