You would need to book and pay for your Covid-19 travel package before you board your flight back to the UK. Your Fit-to-Fly test kit will be with you at least 4 days before your travel out of the UK. 4 Medical Testing will ensure that your day-2 and day-8 testing kits (self-swabbing kits) arrives at your home (designated) address in the UK on day of your arrival in the UK. Your TTR test kit will reach you on your day 5 as required by the UK government.
