So yeah, a certain proportion of those who operate cash-only businesses - gardeners, builders, whatever - fraudulently under-report their earnings. A proportion of those will also fraudulently claim benefits, though less than you might think - if you're going to do benefit fraud it makes more sense to do it on a larger scale, you're risking the same penalties and getting more income. But in any case, it's misleading to talk as though this were in any way specific to prostitution.