Regardless of hunting I don't think the fox population will ever reach a level where they are an effective control for rabbits. If the hunt doesn't get 'em the chicken farmers and gamekeepers will. The hunts often employ measures to enhance the fox population in order to ensure continuity of their sport.

For the record, my own position on hunting is that for the majority of participants it's a great opportunity to engage in some out of the ordinary exhilarating riding, but killing the fox with a pack of hounds is an unnecessarily cruel part of the sport. I'd be in favour of a ban on hunting foxes with hounds, but would not like to see hunting banned altogether. The fox can easily be replaced by a drag scent and the inginuity of the trail-maker can make the chase just as exciting.