Liberals of all ages have to find the time, make the effort and given the reason to come out and vote in the upcoming elections.
There a couple of things that are coming quite apparent in elections in democratic countries:
1) Incumbents continue to do better than most polls predict. This has always been a truism given the perks and visibility of elected office.
2) Conservatives also do better than polls predict. Liberal voters don't vote in the numbers pollsters predict.
So putting these two observations together, the UK vote outcome is not surprising.
I think the problem lies in the fact that liberal votes don't show up at the polls. Look at what happened in Scotland where the turnout was over 70%. Conservatives got trounced. Same in Alberta Canada, voter turnout up almost 10%, conservatives lost a 44 reign.
Come to think of it, i believe the Republicans figured this out years ago hence all the voter suppression laws.