Well reasons would differ widely between election to election, due to the changing nature of politics and parties.

Let's take the 1983 election, the worst result for Labour, as an example: Firstly, the Conservative government was riding a wave of popularity stemming from the prior year's South Atlantic War. However, there was also increases in consumer spending and falling interest rates prior to the Conflict. According to a study a couple of years later, these macroeconomic benefits would lead to a surge in support for the incumbent government^(1).
Furthermore, 1981 saw a split in the Labour party where more moderate members split off to found the Social Democratic Party (which had made an alliance with the Liberal Party by the 1983 General Election) after a leftward shift due to a change in leadership. This would have also seen a split in votes away from Labour, particularly amongst more moderate voters.

Finally, the BBC suggests that the election campaigns of both the SDP-Liberal Alliance and the Labour Party were underfunded, especially when compared to the Conservative Party^(2).

1: Fletcher, T. et al. (1987) 'Government Popularity and the Falklands War: A Reassessment', British Journal of Political Science, Vol. 17(3), pp. 281-313

2: http://news.bbc.co.uk/1/hi/uk_politics/vote_2005/basics/4393313.stm