Current UK COVID-19 restrictions make it unlawful for Britons to travel abroad for holidays.
Residents of England wishing to travel abroad (except to Ireland) must complete a declaration of international travel confirming that they have a "reasonable excuse to travel abroad."
Different restrictions apply to residents of Scotland, Wales and Northern Ireland - see the FCO website for up-to-date information on these.
The restrictions stated below apply to persons traveling directly from the UK to the various countries for a holiday. Different rules apply to persons transiting these countries, or entering via a 3rd country. Consult government websites as relevant.
Spain:
The FCDO advises against all but essential travel to Spain including the Balearic Islands but excluding the Canary Islands.
Since December 2020, Spain has banned travel from the UK by air or sea except for Spanish and Andorran nationals and permanent residents. These restrictions will remain in place until at least 30 March 2021.
France:
Beginning 12 March 2021, visitors from the UK no longer need to have an "essential reason" to enter France.
All travelers from the UK aged 11 years and above must present a negative PCR test result dating from no longer than 72 hours prior to arrival, self-isolate for 7 days upon arrival, then take another PCR test.
Arrivals from the UK must complete a sworn statement certifying they are not suffering from COVID-19 symptoms and have not been in contact with confirmed cases in the previous 2 weeks.
Italy:
The FCDO advises against all but essential travel to all of Italy.
UK residents traveling to Italy must complete a self-declaration from the Ministry of the Interior.
From now through 6 April entry into Italy is permitted only to residents or those with "absolute necessity" and only for visits shorter than 120 hours. A written application to the Ministry of Health is required.
Prior to 6 April, persons wishing to fly from the UK to Italy must present the airline with a negative COVID-19 rapid-antigenic or molecular-swab test taken no more than 72 hours before arrival in Italy; and also take a second such test within 48 hours of entering Italy (at the airport upon arrival, for air passengers).
Persons returning to the UK from Italy must report to their local health authorities upon arrival, and self-isolate for 14 days.
To be safe, check the Italian government's COVID-19 travel information for late updates.
Greece:
The FCDO advises against all but essential travel to Greece except for the islands of Rhodes, Kos, Zakynthos, Corfu and Crete.
Non-exempt persons returning to the UK from ANY part of Greece must self-isolate upon arrival.
The FCDO is not advising those already traveling in Greece to leave at this time.
UK nationals are permitted to enter Greece if they are permanent residents of the UK, Greece, another EU/EFTA state, or one of the following: Australia, New Zealand, Israel, Russia, South Korea, Thailand or UAE.
Persons entering Greece must complete a Passenger Locator Form at least 24 hours before travel. Violators are subject to a 500 euro fine and/or refusal of entry to Greece.
All visitors must provide proof of a negative PCR test taken with 72 hours of arrival in Greece.
Persons entering Greece from the UK will be asked to undergo a rapid COVID-19 test on arrival, and if test is negative, self-isolate for 7 days (if positive, at least 14 days) and in either case take and pass another PCR test at the end of the self-isolation period.
Comment: I think I'd wait a while before booking an expensive 2021 summer holiday to any of these countries...
