I think this is the case with Lots of chains They expand too much and lose quality, I remember going to wash a when it first opened about 10 years ago and it was great, I went recently (Ish) and the food was awful, all at fridge temperature and very blandI used to work at one of his restaurants in London for many years.
Met Jamie Oliver few times, nothing bad to say about him - nice, friendly personality.
The place started to go down waaaay before Brexit campaigns even started.. So that is a very lame excuse in my opinion.
Main reason for this failure was being too greedy. Opening restaurants one after the other , instead of focusing on the existing ones. Quantity over quality. I don't know about other restaurants, but where I worked we were massively understaffed - Great they would save every penny on chefs, waitresses.. Hence the overcooked/undercooked food, ugly presentation, stressed waiters, angry customers and so on ..
I can't blame Jamie for this, but whoever is overseeing his business and management -they need to be sacked.
I also think that Area managers were pushed to the limits to hit the targets and sales .
It is not ''cool'' to have restaurants on each corner with crappy quality. He should've stopped at having 5 restaurants - couple of London locations and couple outside of London. I guarantee you if this was the case there would be still long lines outside of his restaurants waiting for the tables. But it's too late, the reputation is massively damaged now.
At the end all of the good, talented chefs started to leave because they didn't see any potential there and company was so desperate that they made kitchen porters into chefs within couple of weeks (literally).