Just look at the chart above showing the aggregate CON+LAB vote in all general elections since 1950. GE2010 saw the big two share down to its lowest level. Now with the emergence of UKIP it could edge down even more.
What this means is that it is possible for a party to win a general election with little more than a third of the GB vote. At GE2005 Tony Blair’s Labour came home with a 60+ majority on just 36.2% of the GB vote. That is slightly higher than the UK vote with also includes Northern Ireland.
It is important to note that all opinion polls are based the GB shares only.