Red-list countries are those the government says should not be visited “except in the most extreme of circumstances”.
There are now only seven destinations on the red list:

Colombia
Dominican Republic
Ecuador
Haiti
Panama
Peru
Venezuela

If you have been in one of these countries in the last 10 days, you cannot travel to the UK.
The only exception is if you are a UK or Irish national, or UK resident – in which case you need to:

Take a Covid-19 test before departure for the UK

Complete a passenger locator form

Self-isolate on arrival in the UK for 10 days in a quarantine hotel, paid in advance

These rules apply whether or not you have been fully vaccinated.
Rates for quarantine hotels are:

10 days (11 nights) for one adult, £2,285
Additional adult (age 12+), £1,430
Children aged five to 11, £325