COVID-19
Update 05.08.2021:
From 8th August, Slovenia and Romania will join Bulgaria and Croatia on the UK Government's Green list for travel. From 30th August, Montenegro is on the UK Government's Red list for travel.
All other countries in the Balkans are on the UK Government’s Amber List for travel.
Full details of the list and the UK Government’s requirements are here: https://www.gov.uk/guidance/red-amber-and-green-list-rules-for-entering-england