To enter Great Britain you now must show proof of a negative Covid-19 test even if you are vaccinated. This is due to their current surge. The result can be from the rapid antigen test but must be taken within 48 hours of your flight time. Before you fly you must also fill in their official “locater form” as well as reserve and pay for a Covid test ahead of time for your first or second full day after arrival. The all-important info can be found here. By the way, London’s BOOTS PHARMACY happily accepts non-British visitors for the rapid test. They have many locations so book ahead through their website.