Vote Labour.
Yes, they're not perfect, but no-one is and furthermore no-one can be. Pick any one of the ten-a-penny idealistic small parties in the country - if they ever wanted to gain enough supporters to run the country (and they don't), they would have to compromise and U-turn every bit as much as the big parties at the top. If you still think your 'pure' ballot would be spoiled by ever voting for a party that represents anything but 100% of your opinions and no-one else's, consider this: politicians LOVE low turnouts. Low turnouts mean that they can spend more and more of their resources courting a handful of swing voters in the centre who decide the election.
The Blairite/Thatcherite wing of the Labour party are in decline for the first time in 20 years. Ed Milliband may be a piss-poor presenter, but his policies are solid, with a consistent theme of forcing markets to work for ordinary people. The Labour party's core values have always been about standing up for ordinary working people - of course they've made mistakes, but that has to be preferable to the Tories, whose explicit goal is to help the wealthy maintain their wealth and enrich themselves further.
The Tory party started out as a broad coalition opposing voting rights for non-landowners and fighting for more tax money to go to the nobility, but now they can disguise their goals as offering people "freedom from government" and "lower taxes", not mentioning the fact that despite their best efforts, the government still broadly protects workers from the whims of the rich, who also still pay the most tax. Labour was founded by working people and even today its (admittedly posh, policy-wonk) leaders are elected by working people. Labour fought for and won the majority of workers' rights, the NHS, free education, social housing, the minimum wage, racial, sexual and religious equality, all of which were opposed tooth and nail by the Tories.
Even if it's just a least-worst calculation, please vote. If your second or third-worst party is going to get in anyway, they won't get any more power from your vote. On the other hand, if your vote makes the difference then you've prevented the worst party from getting in and bought a small piece of negotiating power to try to change the party that now wants to keep your vote.
Whereas if I refuse to pay my TV tax (and ensuing fine) to pay the wages of twats like Jeremy Clarkson his £1m salary I'm gonna end up serving time in prison.
This problem, like it or not, can only be solved internationally. If the UK unilaterally forces the thieving bastards to pay all the tax they owe, on time, no exceptions, then the majority will move and we lose the tax that they do pay, while the US or Switzerland gets it instead. The only real solution is an international treaty between the EU, US, Aus etc. promising to all crack down at the same time, meaning that the rich can either pay up or move to Dubai where it's illegal to drink alcohol and fornicate, or Beijing where they can be arrested for criticising the government. No single UK government can make this happen on their own, but you can bet that Ed Milliband would if he could. Would the Tories?
I can see you've put quite a bit of thought into this, but the fact remains that even if Labour are now moving away from Blairite/Thatcherite policies (and I'm yet to be wholly convinced they are) they're still in the same neo-liberal neighbourhood as the other parties in terms of economic policy. They might not be as extreme in their views as the tories (or that joke of a party, ukip), but they're still heading in the same direction, albeit more slowly. Under Labour, there will still be privatisation of the NHS, there will still be a push for forcing people into zero hour contract "jobs" to manipulate unemployment figures while boosting corporate profits and undermining the minimum wage. There will still be an intentional and deliberate transfer of wealth from poorest to richest. I refuse to vote for that.
This shows where the parties stand. You'll notice all but one are in the same ballpark. I don't care if everyone else wants to vote for a party that's going to get in or not. I'm sticking with my principles and voting for the party pulling in the opposite direction to the cliff this country is heading towards.