Every time the political situation in Britain is not straightforward, there are voices saying that there will be a return to national government, last seen 1931-45.
Of course anything in the future can happen, but it seems highly likely that following the 2015 General Election in May this year either the Tories or Labour will emerge as the single largest party in Parliament with the other in second position. By British convention the party with the largest number of seats will be invited to form a government, and if not successful the remit will fall to second party. It is highly improbable that neither Cameron nor Miliband would be unable to form a government which would not immediately fall in a parliamentary vote of no confidence. UKIP and the Northern Ireland Unionists would lean towards the Tories, while the SNP (likely to emerge as a major player in 2015) along with Plaid Cymru and the Greens (if they win seats) would favour Labour. The Liberal Democrats would get into bed with anyone, and it is unlikely after an electoral beating in May 2015 they would want a second general election.
In any event, a coalition only between the Tories and Labour would be, to borrow German terminology, a “grand coalition,” not a national government. Such a government, if only because it was so unexpected and would deny the voters any say in the direction of the country, would be unpopular and would soon haemorrhage support to the left (mainly the Greens, but also the SNP in Scotland) and to the right (UKIP). Recent opinion polls put support for a Tory/Lab coalition as the preferred outcome of a hung parliament at a mere 9 percent. Anticipating low levels of public support and votes falling away, a grand coalition would be the last choice for both Cameron and Miliband, or their successors.
A “national government” is not impossible, but it is highly unlikely.