So here are my thoughts on electoral systems: When considering which electoral systems are going to best represent the people of a country, there are many important issues that have to be considered (in no particular order except that in which they came to mind).
The "first past the post" system currently employed in the UK works very well in this respect in that approximately 70,000 people in each constituency have a designated MP for their geographical area who they can go to with their problems. Any move towards PR will dilute this relationship to some extent. The first past the post system also forces the parties to tailor their candidates to the local political climate and so encourages a broad range of political views within their party. By example, Dennis Skinner, famously left-wing labour MP, is presumably elected because he reflects opinions in a left-wing constituency. In a party list system, where would people like Dennis Skinner come, presuming that the party leaders would rather have sheep towing the party line? Where would all the candidates come from, and how would the citizens know who to complain to? The "single transferable vote within party list" system that would give the citizens a say in where Dennis Skinners end up would surely favour candidates with a high public profile rather than quietly effective politicians which I suspect would be bad for effective government. A move towards the single transferable vote within present constituency boundaries causes no loss of the local accountability which is the one great strength of the current system.
Does your vote "count"?:
"First past the post" I believe is a misleading term, which applies only to westminster seats, and not to the situation in constituencies. A more accurate term for what happens in constituencies is "winner takes all". There is no "post" in percentage points past which you are guaranteed victory. The person with the most votes represents everyone in that constituency regardless of whether they love, tolerate or hate them. If you favour one of the smaller political parties, the greens for example, your vote is always going to be wasted under the present system. Anyone with any sense who feels the greens best represent their political views should ditch idealism and vote for the candidate who is closest to their standpoint and has a chance of winning. THIS IS NOT DEMOCRACY. If you walk into a voting booth and intelligently choose to vote tactically, your true opinions and beliefs are being suppressed. Okay, so it's you that is suppressing them, but you should not be forced into that situation in a truly democratic country.
The "single transferable vote" system destroys the need for tactical voting. You can rank everyone on the ballot according to how well they represent your views. If your first vote was for a party which didn't get many votes, your second preference may be crucial in deciding which candidate squeezes past the 50% mark needed for victory. Your vote counts at last. Oh, and if you don't feel you have enough information to engage and make a decision, you have the option of tossing a coin to determine the rankings of the other candidates before ranking the BNP last.
No longer would liberals stay at home or vote tactically in a labour-tory marginal. They can go out and vote liberal 1st and find out just how many people around them truly share their views. The same would go for tories in labour-SNP marginals and... well... everyone everywhere.
The Scottish parliament election system has the first past the post system with regional party list top-ups which has led to coalition government. It can be argued that coalitions lead to instability and damage the ability of governments to concentrate on running the country. It can also be argued however that a broader range of views (and citizens) are represented in coalition government, and that lesser coalition partners can be a moderating force preventing government flip-flopping to the the extremes of the political spectrum every 4 or 5 years. No country is going to be well served by 3 years of every 5 year cycle being spent undoing the policies of the previous government. Fortunately(?) it seems that British politics has lost its extremes - at least within the main political parties - and this would likely not occur.
A problem occurs however when the coalition partner is a party further to the extremes of the political spectrum. Try to imagine a situation whereby the percentage of the national vote is reflected in seats in parliament and the conservative party have no majority but can get legislation through if they make concessions to UKIP and/or the BNP. The voters have little say in which party is chosen as a junior coalition partner, thus the good intentions in choosing PR to create a more representative democracy are destroyed by an undemocratic negotiation amongst party leaders. Should a coalition situation arise under the single transferable vote system, the party leaders could theoretically refer to the second preferences of their supporters in judging where they would hope the party seek alliance.
In conclusion, the British electoral system would lose nothing and gain a great deal from adopting the single transferable vote. The only argument against it that I can see is an extra few hours counting votes in marginals on election night. I can live with that.