United Kingdom is currently partially open to tourists and quarantine is required depending on where you’re coming from. The UK has dropped all plans for a covid vaccine passport. Source

Requirements

Quarantine is required at government designated site or self-isolate at home. Travelers arriving from green-listed countries are not required to quarantine unless a mandatory COVID-19 test taken on or before the second day after arriving in the UK returns a positive result.

Restrictions

As of July 19th 2021, masks are no longer a requirement indoors. Restaurants and bars are open as usual. Updated September 12th