It's a conglomeration of factors, yes the Breweries take a hefty old chunk, but you will find that they charge extra on a Barrel of their Beer as part of the lease costs for the Pub in a tied House. As to soft drinks, spirits & guest Beers the Landlord/lady can shop around.
The Government of the day screws an extra few bob in taxes & charges out of the beleaguered tenants & there is no way they'll repeal these things as they are an out & out cash cow to fill government Coffers.
The Smoking ban certainly hasn't helped, but it isn't a major contributor to the situation, pub attendances were dropping off before it was applied. But any hardship for this can be laid squarely at the door of Brussels as this is most certainly one of their edicts, but UK's implementation is by far one of the most stringent.
MEPs call for 'unrestricted smoking ban' in Europe | EurActiv
Finally the people themselves are responsible for the decline of the street corner Boozer, they haven't as much money to chuck around on "Luxuries" like going out for a few scoops with friends, so they stay at home drinking cheap imported Beer, or cheap British Beer bought in the local supermarket & this is an ongoing trend so the demise of the local will continue.
So like I said, it's not purely the Breweries there are plenty of factors involved in the downturn in the fortunes of the "Local".