By way of a rider to this reply, perhaps it would be useful to add for those who don't know the U.K. situation at all that these legal gun-owners are still a pretty large group. Shooting field sports have, in fact, been flourishing in recent years. The three large commerical shooting estates (mostly pheasant) within a few miles of where I live have never done such good business: for many city businessmen, a day's shooting has replaced the golf-course as the must-do networking recreation. And all my farming neighbours have at least two legally-owned guns. While grouse shooting and deerstalking are still rich men's sports, there are many other forms of shooting field sport which are accessible to more or less anybody at reasonable cost.
You can own a .50 calibre rifle. However, at my rifle club, I was told that you need a valid firearms cert with a 50 cal entry on it even to fire someone else’s!