All visitors to the UK should be put into 14 day quarantine until covid-19 has been dealt with around the world. There is no point to a lockdown if people keep bringing in the virus from other countries. A 14 day quarantine would ensure people arriving don't transmit it.
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Having a 14 day quarantine in place should mean once the virus is dealt with in the UK we can return to normal in the knowledge that the virus will not be brought in from abroad. Lots of countries have put quarantine in place already. The UK should stop waiting until the situation gets worse before putting in new rules.