I seem to recall that the horse chestnuts are not suffering from disease, as such (although there is some evidence of a weeping canker, or something like that, in some trees) but that the leaves are infested with leaf miners - the larvae of a particular moth. This causes the leaves to turn brown and autumnal-looking early, and the leaves drop prematurely.
This, in itself, does not apparently harm the trees, although there may be as yet undiscovered consequences. But stopping the spread of the moth is difficult, and it may be that "disease" is used as a shorthand for getting rid of some lovely mature specimens.
As for the wholesale destruction of trees whilst claiming to act on behalf of the population at large seems somewhat perverse. Nick Clegg is, as Philippa suggests, MP for Sheffield Hallam. (I think he has a little more spare time these days, so may be willing to help )