England, as well as Scotland, has updated its Travel Corridor list, removing four countries from quarantine exemption. This means that arrivals from Denmark, Iceland, Curaçao, and Slovakia will have to quarantine upon arrival where they once previously were exempt. This new change will go into effect at 04:00 British Summer Time on September 26th. At the time of writing, Wales and Northern Ireland have yet to update their lists but they will likely follow.
It is also important to remember that transit-travel counts as an Arrival from an exempt/non-exempt country. For example, if you fly from an exempt country, such as Greece, but have a stopover in Copenhagen, self-isolation is required as Denmark will not be exempt.
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Change will commence at 04:00 Saturday
Travelers arriving in the UK from Denmark, Iceland, Curaçao, and Slovakia from 04:00 on Saturday, September 26th, will need to self-isolate for two weeks. This is a result of the countries being removed from the Travel Corridors list.
This means that any UK residents vacationing or on business in any of the four countries will either be required to return before 04:00 tomorrow or else face 14 days in self-isolation. With less than 24 hours remaining, this, unfortunately, might be a rude awakening for some, leading to a last-minute scramble to get home.
Change based on health data
"There has been a consistent increase in COVID-19 cases per 100,000 of the population in Iceland, with data showing a 921% increase in newly reported cases over the past week. In Slovakia, there has been a consistent increase in the weekly case rate of COVID-19 over the past 4 weeks, with a 115% increase in weekly cases per 100,000 between 2nd and 23rd September...There has also been a 508% increase in newly reported cases per week in Denmark over the past four weeks, and a 481% increase in newly reported cases over 7 days per 100,000 between 2nd and 23rd September in Curaçao."
All lists are subject to change
The conditions remain fluid, and as a result, are subject to change and may not accurately reflect the situation depending on the time of reading. As such, some countries may be added to the list of exemptions, while some may be removed.
Below are links to the official government pages which list exemptions and their effective dates. There may be slight variations between each UK country.