The UK uses an electoral system called first past the post. In the 650 constituencies that make up the House of Commons, the candidate that wins is the one that gets the most votes. A majority of votes is not needed, the candidate just needs to come first, hence the name first past the post.

In 1983 the opposition to the Conservatives was divided between the Labour Party and the Social Democratic Party-Liberal Party alliance. The party with the most votes gets a disproportionate amount of seats due to the electoral system. Labour and SDP-Liberal alliance got 27.6% and 25.4% respectively, a total of 53%. The Conservatives got 42.4% but 60% of seats.

The Conservatives had been highly unpopular throughout 1981 and the beginning of 1982. By February opinion polls gave them their first lead but other polls still put them behind Labour. This was the state of play on the eve of the Falklands War which broke out in April 1982. By the end of the conflict in June, the Conservatives had a lead of between 15%-20%. The victorious outcome of the war transformed the Conservatives' standing in the eyes of the electorate. Their was a surge in patriotic support for Thatcher as the lady who won the war. The Conservatives retained this lead up to the 1983 election.

Two major factors stand out as the causes of the Conservatives' 1983 win. First, the electoral system usually gives the party with the most votes a majority of seats in the Commons. Secondly, the Falklands War transformed a roughly even distribution of voting intentions between Conservatives/Labour/SDP-Liberal into a strong Conservative lead.

Michael Foot became leader of the Labour party in 1980. He was well liked in the party and was a good public speaker. Holding views such as unilateral Disarmament, and withdrawal from the EEC, he should have been seen as being on the left of the Party.

He was intended as a compromise candidate, yet it wasn't a compromise that really satisfied anyone. The right of the party eventually split to form the SDP, and the left under Tony Benn continually tried to undermine the leadership, and eventually managed to force through the 1983 manifesto.

A disastrous deputy leadership contest which pitted Dennis Healy against Tony Benn dragged on for months. Dennis Healy's campaign was disrupted by the various left wing groups that existed on the left of the Party. All of which was prominently covered by the media.

One theory goes that 1983 manifesto was never intended to be allowed to go through in it's original form. It was expected that more moderate voices would force though changes that would have made it more electorally acceptable. Instead the opposition it met with was to the manifesto as a whole, and the expected moderate revisions never happened. Essentially the moderates walked away and let the manifesto go forward unopposed.

It's important to remember when talking about the 1983 manifesto that the description 'longest suicide note in history' was coined by a labour MP Gerald Kaufman and not by the Government. In short the labour party actively took steps to lose the election.

A very good source is the 1990s documentary 'The Wilderness years'

https://www.youtube.com/watch?v=3XrO72C1WQ0&list=PLNryPuGhNcDYIH6jpd58Rl57Y_IUGoQ-3