Non-brit here. Looks to me like Labour just refused to deal with the fact that the UK populace's public opinion has been drifting rightwards for the last 6 years minimum. They just didn't seem to deal with any of the issues that either the working class or hard left actually have, instead harping on easy points like the NHS. Losing centrist votes to the Tories as those people drift right, (some) working class votes to the UKIP on the immigration and Europe issues (not that they'd probably want those votes), left-leaning votes from idealists to the Greens and all of Scotland to the SNP.
I'm a left-leaning Guardian reader, but I can't figure out why on Earth I would have voted Labour. The memory of the Blair years when they were as bad as any Tories (lies, war, bank deregulation & meltdown, unchained immigration etc) is still in my mind. So what are my options? If I want to shift the country properly leftwards and vote with empathy then I'm voting green (what I would have done, most likely). If I want to vote out of self-interest and 'a safe pair of hands' for the economy I'd leave the Tories in power. Yes they can be nasty, especially to the most vulnerable but they tend to do exactly what they say on the tin, so they're at least honest. I know what I'm getting with them. If I'm okay with being selfish (I'm not) that's probably my vote. If I'm Scottish I vote for the SNP.
Watching from afar as a non-Brit with mostly British friends and workers I just don't get the current appeal of Labour - and I'd like to, I would. Every vote I've ever cast has been for the candidate on the left. As for Milliband, well, what can you say about his leadership. All the charisma of a wet rag. Message neither delivered nor received. Not sure where Labour go from here.
I've just seen
this quick reaction piece in the Guardian having written this comment and I think it nails the basic reasons. It's a failure on all almost all fronts and it's embarrassing that they were as complacent with their strategy as they appeared to be. Let's see what the final numbers are, I suppose.