On a slightly more cheerful note - what about this for a thesis:
The elite in Britain are extremely successful at maintaining their power and influence pretty much regardless of the government in power, and effectively preventing any substantial redistribution of resources by the Labour party purely on the back of an electoral mandate.
The times historically in which there have been substantial gains by the common people have been (pace the post war settlement) when this elite has felt threatened with all out revolution.
Generally, given the quietism of the British people, this mainly happens when there are serious revolutionary movements elsewhere in Europe, which could - potentially - spread to Britain.
Clearly, there is the potential for this to be a moment of crisis throughout Europe (indeed it is already shaping up that way) - cf the wave of revolutions that ended feudalism in a very few years.
So potentially, all we (those of us on the left) need to do is hope that things kick off big style in Europe, and then that there is some sympathetic movement here - albeit on a smaller scale - but enough to seriously stress the elite.
I know its a long chance, but hell, there's nothing like optimism 