During the 2021 lockdown, the UK clamped down on international travel. But on May 17, non-essential international travel resumed in England, Scotland and Wales under a risk-based "traffic light" system, dividing countries into "red", "amber" or "green" categories. This system was implemented in Northern Ireland from May 24.
https://www.cnn.com/travel/article/uk-travel-covid-19/index.html