If they're ordinary petunias and not trailing ones, this is exactly how I'd expect them to look by now - they always collapse over, have lots of bare stem towards the base, and are generally quite sticky to touch. Its a bit late to cut them back I'm afraid, by the time you get any regrowth, it'll be September, but as they're only annuals anyway, you could risk it, giving them a feed at the same time. If we have a mild autumn, you might get a few flowers in late September/October.