People arriving from Singapore and Thailand in England and Scotland will not need to quarantine from Saturday morning, the government has said.
They have been added to the list of “travel corridor” countries.
But travellers coming from Slovenia and Guadeloupe will have to self-isolate for two weeks.
The two countries have also been added to Wales’ quarantine list, while arrivals there from Gibraltar and Thailand will not need to self-isolate.
The changes come into force at 04:00 BST on Saturday.
The Department for Transport (DfT) said there had been “a significant change in both the level and pace of confirmed cases of coronavirus” in both Slovenia and Guadeloupe.
Data from Slovenia shows that its seven-day rate of cases is 29.1 per 100,000 people, up from 14.4 in the previous seven days.
The rate for Guadeloupe has risen more than six-fold in the past four weeks, the DfT said.
When a country’s rate rises above 20, the UK government considers imposing quarantine restrictions.
The decision to remove quarantine restrictions for arrivals from Thailand and Singapore is unlikely to lead in a surge of people from England visiting as both countries are only allowing people to enter for a limited number of reasons, such as if they have a work permit or are the spouse or child of a resident.
Travellers who do not self-isolate when they are supposed to can be fined £1,000 in England, Wales and Northern Ireland, or £480 in Scotland.
They can be fined up to £3,200 in England if they do not provide accurate contact details, or £1,920 in Wales.