Brexit and the waning days of the United Kingdom

[Josquius]

Yeah, 40 million are bred in captivity and released en-masse every Summer.

They're also a bit more recent than the Romans. The Romans brought some over but they largely died out until the Normans brought some over again. The Norman aristocracy then set in place laws to protect them and ensure they remained in place for hunting.... but nonetheless these Norman pheasants are now basically gone, they were mostly replaced by Chinese imports in the 18th century.

Stop shooting and no way would they become pests and take over. I don't think they'd completely die out over any remotely human time scale either, but they would become pretty rare. Needs noting even those breeding out in the wild are fed over winter by game keepers.

Deer are quite a different matter as they are naturally evolved for the British landscape where they're also missing natural predators.
Pheasants are from quite a different place and make an excellent meal for a fox.