A useful list of which countries are welcoming foreign visitors, and which still have restrictions.
Most countries require a negative Covid test and completion of a passenger information form, and some ask for evidence of vaccination. The FCDO website (www.gov.uk/foreign-travel-advice) has a clear summary of country entry requirements in more detail.
Fully open, no significant restrictions*
Argentina, Bolivia, Brazil, Colombia†, Costa Rica†, Cuba, Ecuador/Galapagos, El Salvador†, Guatemala, Guyana, Honduras, Nicaragua, Mexico†, Panama, Paraguay, Peru, Uruguay
*for vaccinated travellers
†no pre-arrival test required
Open, but with some restrictions
Chile (arrival test), Venezuela (arrival test)
Not open
Falkland Islands