Companies fold all the time can't really link the high st to Brexit too much it's a factor but it's been in decline for decades. It's a number of more influential factors for me, high rates/rent parking charges for town / city centers add to that the time it takes to visit the town centers now, and finally people like me who hate shopping, I only go under duress and left to my own devices would never go other than for clothes as you can make sure they fit.
The retail sector won't be around in it current guise for alot longer, I think they will create a number of viewing spaces where you view items before you buy, then shipped to your home directly.
Councils are haphazardly running around to redevelop these spaces into social areas already, I think most will be populated by apartments and restaurants before you know it.