"Our first past the post system of voting will mean that for a relatively high number of votes, UKIP won't get a correspondingly large number of MPs, which will leave a lot of people frustrated. "
I've been spending the last month or so repeating this to anyone who's interested (and a few people who aren't, probably) -- my hope is that UKIP manage to hold on to approx 15% of the vote share and get essentially nothing for their troubles.
The preferred electoral system for UKIP is what is known as "AV plus", but should really be called the "mixed-member alternative vote". In this system, each voter is invited to cast two types of vote. The first vote works exactly like the alternative vote, ranking candidates in order of preference, and then the count proceeds by totting up first preference votes, seeing if anyone has made 50%+1 already, and if not then eliminating the last-place finisher and redistributing their votes according to second preference, and so on until someone has reached 50%+1. This will always happen.
However, this also only fills a half of the house. The voters' second vote is for a party they would prefer. These votes are used after the AV part of the vote has been completed, and what follows is MPs being chosen from the party list to fill the house until the overall representation is proportional, or as close to it as possible.
The advantage of this system is that it preserves a constituency link, at least for half of the people voted for, while also avoiding situations where MPs can win on a very tiny minority of the popular vote (at little at 29.4% at the 2010 election, and that record is likely to be beating this time around). Additionally, the smaller parties that will often be shut out at the first round of ballots still get a chance to have some representation anyway, in proportion with their total support.
The big problem is that unless the divide between directly elected MPs and the party list MPs exactly corresponds to the divide between houses, then you have two classes of MP in the House. I don't see how this can be resolved in any way other than splitting the houses (as in, this is a great way to combine electoral reform with Lords reform), and then retaining a modified form of the Parliament Act that establishes how the balance between the two houses should work. If the MPs from both parts of the election have equal power, you can see that this would be messy. One lot is elected basically to "make up the numbers" but voters have no control over the list (the perennial problem with any form of 'pure' PR -- you can't hold individuals to account), while another has a direct link to the constituents who voted for them> Why should they be equal? AV Plus, and equivalent systems, demand a House split and an addressing of how the balance should work, so is difficult to implement.