I've been giving some thought to the result in Barnsley Central, and what it all means.
One important fact is that turnout was way down on the election. This perhaps reflects the fact that by-elections are often protests against the sitting government and that it wasn't going to affect the math in parliament.
The reason I think that UKIP came 2nd was partly that they worked so hard, but also that people knew that they didn't have to vote tactically because it wouldn't change the status of the government. Win or lose, the LD/Con coalition was going to remain in place. So they could vote freely. UKIP doubled their vote and nearly tripled their share of the vote.
And if you look at EU elections, UKIP also score highly because with the PR system, people can vote freely.
This is, of course, why the 2 main parties are so against it. If they ever dramatically screw up (and the expenses scandal was such a screw up), people won't just think "he's a scumbag, but I'll vote for him because I don't want the opposite guy's party in", they'll find another party that they find nearly as agreeable and vote for them.