Look, the food industry is oversaturated, and restaurants change hands or close down all the time, even before Brexit. With so many choice and variety, it's stiff competition and people tend to go for the cheaper options, unless they want to pay for quality.
Jamie Oliver's restaurants, seem to sit in the middle, neither cheap or good enough quality for the upmarket customers. Sure there's middle ground, however it's flooded with restaurants like Frankie and Bennys, TGI Fridays, Nandos and etc. All of whom are competing on deals and price. So why would customers go to Jamie Oliver, if they can go to a cheaper alternative with the same level of quality?
Another point is, I wonder what the overheads were, as I doubt any of the closed restaurants owned the property. I could guess that rent alone, would be over £100,000-£200,000 or more a year for city units in prime locations. If business wasn't good, they'd not meet that target, and you got other expenses, taxes and wages to pay on top. I would guess that you'd have to pay well over £200,000-£300,000 a year.
The restaurant business, actually the whole food industry, is one of the most challenged industries in the UK. Simply because it's oversaturated, with far too many outlets. This means, that the biggest businesses, offer the best deals and out price the smaller guys, as they can financially afford to, but is very risky because small businesses would risk it, by offering more for the price like bigger portions. With so many small food businesses, each slice of the pie shrinks drastically.
One type of business to stay clear of, is the food industry in current times. You are better off investing in other sectors or buy property.