Why are foreigners still allowed to travel in and out of the UK? Why bring foreign viruses here?

we in Australia  dont allow  travellers  from abroad  while covid  is rife..our borders are shut ..still not brought all our aussies home from over seas 

The UK is bound by the UN to accept refugees from France

If they've already "brought" virusus here, sending them away won't help, will it?

What makes you think a virus won't travel just as happily on or in a returning British citizen?

Because life would be bloody uncomfortable without imports and exports

The virus is already firmly entrenched in the UK - the risk is higher to travellers.

Although the UK Government's failure to limit incoming travellers, and mandate quarantine for all on arrival, is laughable.

So I must be a foreigner then as I have travelled abroad from the UK many, many times. 

Or even better....nail or weld stuck all doors to people's home and in three-four weeks time there will be no virus anymore.