Fifty years ago , the UK had two big political parties, Labour and the Conservatives, each of wihch were supported by about 50% of the population.
That situation no longer holds, fewer people feel alliegasnce to either of the big parties, and in the last nationwide elections held under PR — the 2004 European elections — Labour and the Conservatives got less than half the vote between them.
We now live in a multi-party society. And while FPTP might have made sense half a century ago, it doesn’t now:
Academics talk about a thing called the “effective number of parties.” In the UK, we have an ENP in Parliament of 2.5 but an ENP in terms of vote share of 3.6. That is an alarmingly high missmatch and as the disparity increases the chances of no-overall control increases accordingly. If the ENP in terms of vote share reaches 4, according to Josep Colomer anyway, “maintaining a majority rule electoral system would be highly risky for the incumbent ruling party” – essentially they lose any real claim of having a mandate (see Helen Margetts’ chapter on Electoral Reform in Unlocking Democracy for more on this). If an election were held tomorrow, it would almost certainly push us over ENP 4. In 2010 it may well happen anyway.