Why could they not just ask all those pilots sat around with nothing to do to start their engines for a few minutes, that'd surely clear loads of the snow?
Or send for as many shovels as you can find then ask those sat in the terminal if anyone minds helping out, im sure there will be plenty of volunteers.
It does seem that Heathrow have decided that the easiest way to get rid off all this pesky snow is to wait for it to melt and save the expense of using their equipment!