WIthout knowing I'd also guess that they would poll their best in very solid Tory constituencies where a win would be very difficult?
It's slightly more complicated than that, but that's certainly one of their main problems. If you look at the green and brown (ignore the red) constituencies - they show the ones in which UKIP saved their deposit (i.e. polled over 5%) in 2010 - on this map...
...and compare to this one...
If they were to make a breakthrough (outside mass Tory base defection in, say, Essex) it might actually be out in the West Country, now that a Liberal vote is a pro-government vote. But I don't think they've ever really made much of an effort in that kind of direction.
