UK travel restrictions: Why isn't the UK on its own quarantine list?

From the BBC:

How is the quarantine list decided?

The Joint Biosecurity Centre (JBC) - set up by the government to monitor coronavirus - works with the chief medical officers of each UK nation and advises on which destinations should be on the list. The decision is often made when 20 or more people out of every 100,000 in a country, or island, are infected over seven days, but other factors are also considered..

From the BBC:

On Friday [1 October], the country recorded 6,968 new cases, slightly down from more than 7,000 a day earlier in the week... An estimate from the Office for National Statistics (ONS), released on Friday, suggests that roughly one in 500 people in England had coronavirus in the week ending 24 September, only slightly down on the previous week.

7,000 new cases a day x 7 = 49,000 cases per week.
(Sure, there are some false positives, but there will be a lot of people who have mild or no symptoms and don't get tested, so let's accept 7,000).
49,000 ÷ population 70 million x 100,000 = 70.
Which is over three times the official limit.

Check: one in 500 people are infected. Out of 100,000 people, 100,000 ÷ 500 are infected = 200 are infected. Assuming each person is infectious for two weeks, it requires 200 ÷ 2 = 100 new infections per week to maintain the one-in-500 ratio. That's a bit more than the first answer 70, but they are in the same ballpark.