It's part of a rewilding effort. They'll be moving exmoor ponies, longhorn cattle, and iron age pigs to this particular woodland as well to recreate the kind of situation that would have existed before the advent of intensive agriculture.
I don't believe there's any evidence of Bison being native to the UK, although there were certainly Aurochs (wild cattle, hence the longhorn cattle), so this isn't a reintroduction as such; nonetheless, it's pretty cool.
Good to see that Eurasian bison are making a comeback. They were down to a single herd in a Polish zoo by the end of WW2. Now there's a halfway stable population in the wild in the old growth Bialowica woods, the first herd has been reintroduced to the Red Hair Mountains in Germany and soon they will frolick around the British countryside and trample terfs and aristocratic nonces under their powerful hooves :Care-Comrade: