Blaming it on Brexit is just PR and spin. The reason why the chain is in trouble is because the food is shit and overpriced.
As PP have already said, the mid-market 'fast casual' sector is very competitve; you've got Prezzo, Zizzi, Pizza Express and ASK all competing in the same pool. You know that 98% of the menu has been mass-produced and vacuum packed in a factory in Slough and shipped out to the branches. But there's an expectation of it being tasty, reasonably priced and consistent which is why people go there.
The problem for Jamie is that all of his competitors just do it better and they aren't hampered by the PR message of the owner espousing fresh and cooked from scratch. By sticking his name over the door and churning out crap he looks like a hypocrite - and worse, a hypocrite who does not actually give a shit about what he sticks his name on.