The Tories are a party for the greedy, the selfish, the narcissistic and the sociopathic; the LibDems for those several sandwiches short of a picnic. UKIP are a party for those who hate and fear, and for those who seek to foment and exploit hatred and fear.
Labour comprise two tribes of essentially decent caring people, locked in a state of permanent civil war which erodes their decency. The only party normal decent folk in England can vote for is the Greens, but it's irrational to vote for the Greens under FPTP, because they can't win.
In Scotland the SNP is composed of exactly the same decent, caring, internationalist and vaguely socialist people as Labour, except that they have one idea - independence - which unites them. This is why Labour hate the SNP so much - because the SNP actually can and do do the things Labour would like to do but can't because their chronic disunity prevents them from ever winning elections. Which is why the SNP and Labour will never be able to unite at Westminster; which means the Tory hegemony will endure the rest of my life.
The largest party in Northern Ireland, meanwhile, make even UKIP look like sweetness and light.
Here's an even bleaker thought: it is those who seek to foment and exploit hatred and fear who are now in the ascendency.
