A PR system in the UK, like in most countries, would all but end single party government. The consequences would be that over time the parties would moderate their views and policies even more, with an eye to forming governing coalitions (crazy small parties like the BNP, UKIP, Greens, would fill the gap at either end of the spectrum). I think another consequence would be that some of the dogma, of the sort that Thatcher espoused and ruined countless lives and communities with, would have to be tempered by political expediency. Also, I suspect that loathed premiers would be less secure. For example, if the last Government had been a Labour/Lib-Dem coalition, it is likely that the Lib-Dems would have had sufficient motivation to ensure that they weren't associated with Brown coming into this election (by either bringing down his Government or pishing Labour colleagues to get rid).