A broken voting system
The UK's First Past the Post (FPTP) does not translate party's vote share (their total votes as a % of all votes) into seats because if you vote for a loser, your vote effectively counts for nothing both within your constituency and nationally. FPTP simply elects whoever gained the most votes whether that person won by 0.01% or by a landslide. Thus a party with many constituency candidates running a close race but coming 2nd has cause to feel there is a democratic deficit.
Alternative Vote (AV) would give 'losing' voters a voice with secondary preferences for example:
London mayoral election system
In 2012 voters received a 2nd option if their 1st choice candidate was eliminated by not reaching the criteria. Their 2nd choices are then redistributed in the 2nd round if that choice was one of the remaining candidates who met the criteria.
Proportional Representation (PR) simply attempts to align % of seats allocated to % of vote share for each party so that there at least national recognition for voters who do not win at constituency level.