COVID-19 is widespread in the UK. An arrival from a country with fewer cases per head may be less likely to have Covid-19 than the average UK resident who’s not been abroad.
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There have been various studies on the incubation period of COVID-19. WHO states on average 5-6 days, CDC states a median time of 4-5 days.
A reduction would help the ailing airline industry and ultimately support the UK economy. Much overseas travel is prevented due to the risk that the government will, with insufficient notice, remove a country from their list of exceptions.
A reduced quarantine period would have higher levels of compliance and be easier to enforce resulting in it potentially becoming more effective.