Ban large public gatherings in the UK. This could be determined by either of some threshold on the maximum number of participants, or an assessment of the likelihood of attendees coming from and then dispersing to a wide enough region. Or both, preferably.
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The virus is clearly spreading fast in the UK. At this point we have two choices: do everything within reason to mitigate its spread, or accept that the death rate could be inflicted upon the entire population, possibly within an unmanageably short space of time.
We do not need a draconian lock down and we must respect individual liberties and economic necessities. But any clearly irresponsible behaviour beyond the necessary must be strongly discouraged or outright banned.