Why is there a participation crisis in the UK?

Low turnout at elections undermines the legitimacy of the results.
Turnout at the 2015 General Election was 66.1%, far below the high levels of the 1970s, where it reached 78.8% in 1974. Of the 66.1% of the electorate that participated in 2015, only 36.9% voted for the Conservative Party.
As a representative democracy, it is vital that turnout is high so that the resulting government can claim to have a legitimate mandate to enact the policies in their manifesto. Turnout tends to be even lower at other elections.
The 2012 Police and Crime Commissioner elections had record low turnout, with an average of just 15%, undermining the notion of elected and accountable police officials.
HOWEVER, low turnout is yet to lead to an actual ‘crisis’ – doesn’t necessarily mean lack of engagement some people don’t vote because they live in a safe seat – real issue is electoral system.

Turnout has also been poor in many recent referendums, undermining the legitimacy of the results.
Only 34% of eligible voters participated in the 1998 referendum on whether to create a London mayor and London Assembly.
Not only did only 50.1% of the Welsh electorate participate in for the 1997 Welsh Assembly referendum, but the ‘yes’ campaign won by the extremely narrow majority of 50.3% – only a quarter of the electorate endorsed this substantial constitutional reform.
HOWEVER, some have had high turnout e.g. 2014 Scottish independence referendum had turnout of 84.5%, reaching 90% in some areas.

Membership of parties has dropped as people have become less involved in politics.
In 2015, only 1.0% of the electorate was a member of the Labour, Conservative or Liberal Democrat parties (compared with 3.8% in 1983).
Labour had over a million members in the 1950s, but by 2010 it had only 193,000. The Conservative Party had almost 3 million members in the 1950s, but this had dropped to around 130,000-150,000 by 2010.
There has been considerable partisan dealignment in recent decades, with far fewer voters strongly identifying with a particular party, loyally supporting them from election to election.
The 2015 Audit of Political Engagement found that only 30% of the public claimed to be a ‘strong supporter’ of a political party, of which only 76% said that they were certain to vote.
HOWEVER, it is now growing e.g. As of March 2017, Labour’s membership has grown to 528,000.