Let's be charitable and go with the organisers number of 400,000 marchers, and less be less charitable and assume that they were all either welfare recipients, public sector workers or other beneficiaries of government largesse.
What were they marching for? In short that the government's budget deficit should be maintained at £170,000,000,000. My long division is not as good it was when I was 9 years old and was really quite god at this sort of thing, but whichever way I look at it, that is £425,000 per marcher. That is not the amount that they would like the government to spend, but the amount they would like the government to spend in excess of what is collected from taxes (an send the bill to our grandchildren). And of course being WR/PSW/OBGL (see above) that means spending the unaffordable £2,125,000 on each one of them and their mates who didn't go to the march over the life of the parliament.
I'd quite like the government to do the same for me, but I don't expect them to. And it should be blindingly obvious to even the most obtuse union leader that there simply aren't enough bashable banks or tax dodging corporations to come close to making up the deficit.