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.
Well they're native and it's going to be carefully managed, so it's not going to turn into a "30-50 feral hogs" situation.
Edit: just looked up the Iron Age Pig, and it's actually the name of a specific hybrid between a Wild Boar and a Tamworth. We do actually have a population of Boar in the Forest of Dean that people keep on complaining about because they turn over their lawns lol