Whilst the U.K. Foreign and Commonwealth Office (FCO) says that nationals are allowed to travel to 67 destinations and won’t have to self-isolate when returning from 66 of them, there are currently only 16 countries without and travel restrictions of any kind for arriving or returning passengers.
For example, the FCO says that Canada is safe to visit, so it is one of the 67 destinations, but returning travelers will still have to quarantine when they come home, meaning it isn’t on the list of 66.
Others such as the island of Bonaire, in the Caribbean, is one place where you won’t need to isolate on return, but it isn’t deemed safe to visit by the FCO (so it appears on the unsafe list of 67). This would make it very difficult to obtain travel insurance.
Other, such as New Zealand and Australia are safe to travel to, but are closed to all foreign travelers.
The Telegraph reported on the 16 destinations which currently make both lists and have few, if any, requirements. 15 are in Europe and the other is Cuba.
In Europe, Denmark, Germany, Gibraltar, Greece, Italy, Liechtenstein, Poland, San Marino, Slovakia, Slovenia, Turkey, and Vatican City are thoroughly open on both sides of the trip.
Three other destinations are open but required Covid-19 testing. Visitors to the Faroe Island are required to take a test at the airport, Iceland requires you to pay for a test upon arrival (or a 14-day self – isolation policy comes into force) and Portugal requires visitors to the Azores or Madeira to show proof of a negative test (within the last 72 hours) or to take a test upon arrival and wait at the accommodation for 12 hours for the results.
As pointed out by The Guardian, there may be more issues to consider too. Often entry might be permitted on other criteria, such as any other countries you have visited in the past two weeks. Switzerland and Italy will not grant entry if you have been in a high-risk country.
Some of the countries on this list of 16, will require an online form to be filled in before arrival at immigration.