In the UK political system, what steps can the opposition take when the government fail to follow their manifesto?
I do not in any way disagree with James K's answer, and he lists the main ways in which the Opposition holds a government accountable. However there is another mechanism he has not mentioned.
If a government pursues an unpopular line, its downfall can come from its own members, or it's coalition allies. UK MPs are elected individually, and their fortunes do not always mirror exactly the fortunes of their party. Those MPs that brought unpopular policies to an end, despite being part of the governing party, can be rewarded at the next election even when their leaders are being voted out.
Since the question is about what the opposition can do, they can talk to and work with potential dissenters in the government, persuading them to vote against the policy. This can be helped by the fact that MPs can have local interests not always aligned with national priorities. If the government promised to support the mining industry, for example, but are not doing so, it is often easy to persuade MPs from mining constituencies to vote with the opposition on the matter.
The can call the government out.
The parliamentary system allows for the leader of the opposition, and other MPs to have time each week to publicly put questions to the Prime Minister. Other Government ministers also have to attend the House of Commons to answer questions. The leader of the opposition can use this time to raise any failures of the Prime Minister (as he sees it) this could include raising the failure of the Government to fulfil a manifesto pledge.
The opposition also has access to the media and can use the media to highlight instances when the Government has not followed their manifesto.
I might note that this is not always considered a particularly effective attack on the Government, especially if the Government is not following a manifesto pledge that had been opposed by the opposition. If (for example) the government had pledged to "raise military spending", but the opposition had pledged to "cut military spending". And then the government broke its promise and cut spending, it would be strange for the opposition to spend much time attacking the government for following an opposition policy.
The opposition is not interested in forcing the government to follow manifesto pledges that the opposition opposes! But if there was a pledge that the opposition decided to support, and they could get enough backbenchers from the government party to vote with the opposition, they could force the government.
After 5 years, the opposition will hope that its actions over the parliament will have changed the minds of enough people that the government loses a general election.
So to summarise:
- The Opposition can't force the government to do anything.
- The Opposition may not want to force the government to follow its manifesto.
- The Opposition can raise failures to follow a manifesto in Parliament and with the media and
- The electorate can decide if they want a change of government at a general election.