Why does UKIP go up in votes and seats?
They coalesced as a party during the EU referendum, much like how the SNP did during the independence referendum. Backlash against the Tories - mostly for the economic downturn that takes place in this wikibox's future combined with unpopular spending cuts - helped them along the rest of the way.
Hard to see turnout going down next time, especially if it's that close. I'd be mildly surprised if turnout doesn't reach 70% next time.
The turnout was depressed because there was a strong urge to kick out the Conservative Party, but no opposition party had a plausible chance of winning a parliamentary majority according to the polls and the papers. Many middle class, Middle England voters who voted Conservative in the last election didn't find Labour, UKIP, or the Liberal Democrats to be a credible alternative, and thus simply didn't show up.