Depends how it's introduced. If it has a threshold to prevent fringe parties getting in to Parliament, then it will possibly stop parties like the BNP, National Front and TUSC into Parliament. I expect Labour would split as in OTL, the SDP wouldn't need the Liberals and might possibly overtake Labour, the Tories would end up splitting over Europe in the 90s. We'd definitely see an SDP or Liberal government I think, either in the late 80s or the 90s. In the 2010s, the Greens gain a sizeable parliamentary presence, as do UKIP.