Given, the brown bear was the traditional UK bear, pre-Roman times.
It is rather hard to imagine them in modern day UK. The population living in areas they are introduced, would def need to be trained and prepared.
I cannot see that happening due to lack of support. Wolves we know as historical fairy tale villains. But in reality they avoid humans where they are still native. Brown bears have more fight than flight when taken by surprise. And don't tend to have the nervousness to avoid the interactions themselves. Hence, why travelling noisily is recommended in areas they reside.
Pretty sure everyone knows the bells on shoes joke.
The biggest question I have, is how those planning this plan to address non human reasons they went extinct.
Because while human effect on the environment was a huge issue. It was not alone. Wolves and bears became isolated to the UK when the European land link flooded (dogger bank) after the ice age.
Much like any isolated population, this lead to genetic diversity issues. Both wolves and bears have large area needs for population diversity. Their territorial nature means groups need large areas. So diversity needs population mixing across those areas.
Back in the 1990 Scotland tried to introduce the wolf. To discover, lack of mixing lead to a high concentration of males vs females. Forcing them to cull the population. Expanding the project nationally may help. But would require safe passage from one territory to all others. And still lead to diversity issues as happened in the past. Bears are much more complicated. As the brown bear is the natural historical resident of the UK. And unlike wolves that avoid human contact due to nerves. Brown bears are much more likely to resort to aggression when humanity blocks its survival goals.
Its not an impossible issue to solve. But is likely to involve consistent investment in introducing new populations. As wolves and bears tend not to buy ferry tickets and use tinder for dating across the EU borders.