Restrictions in response to coronavirus
You must complete an online Passenger Locator Form (PLF) at least 48 hours before your arrival in Greece. The form is online, in English. Once you have completed the form, you will receive a QR code (which you can display either on your mobile phone, or on a printed piece of paper). You will need to show this code to the Greek authorities on your arrival in Greece.
You must wear a face mask at all times when on an aeroplane or ferry travelling to or from Greece; and whilst at airports.
When you arrive in Greece, the Greek authorities will scan the QR code generated when you completed the PLF form, and may direct you for health screening (including testing for coronavirus). The Greek authorities’ instructions will differ according to whether you arrive by land, sea or air. Check out the requirements that will apply to you on the PLF form page.
The Greek authorities may require you to undergo testing for coronavirus, as part of health screening after you arrive in Greece. Any passenger entering Greece is liable to be asked to undergo a test, but you are likely to be asked if you have arrived from a country outside of the EU (including the UK), either directly or via indirect flights. If you are a British national who lives permanently within the EU, or have been in another EU country for some time before travelling on to Greece, you may be asked to prove that you have not recently travelled from the UK.
After testing, you will need to self-isolate at the address given on your PLF form, filled out prior to your arrival in Greece. You will be contacted by the Greek authorities with your test results, and given further instructions (e.g. whether you are free to leave your accommodation, or must self-isolate for a longer period). If your test result is negative, you will no longer need to self-isolate. If your test result is positive, the Greek authorities are likely to ask you to self-isolate in government-provided accommodation for 14 days.
You must comply with all instructions given by the Greek authorities. Failure to do so may result in you being refused permission to enter Greece and / or legal enforcement.
You should also be aware that if other passengers on your flight subsequently test positive, you may be subject to further quarantine/ self isolation requirements. These will be mandatory and you should comply with the Greek Authorities’ requirements.
British nationals are not affected by the European Union’s measures to close EU borders to non-EU country nationals. If you’re a British national, you can still enter Greece, but you must comply with the Greek authorities’ requirements, including on testing and self-isolation.
full detail here