I'm not supporting the idea myself, just responding to a quote that this was a BoJo/Cummings idea, but it is exactly what we are doing at the moment and what other countries have done. With the evidence from China, as soon as one case is confirmed to have spread within your borders then if you don't agree with the theory you should be advocating draconian travel bans & quarantine measures such as was observed within Wuhan. Of course everyone will say this is absurd, and it is, but that would be accepting that you are willing to allow the theory for some time, then declare war on it.