Would love to know what the legal basis for these stops are, as it is not immediately obvious

On what exact legal basis can the officers here insist on cars stopping?

(ie, not just ask nicely)

Anyone? https://twitter.com/TVP_Specials/status/1252510795356876800

Appears there is a general statutory power to stop vehicles - and statutory powers to ask for certain information: ss 163-5 of Road Traffic Act 1988
http://www.legislation.gov.uk/ukpga/1988/52/contents

Presumably this is being combined with the Coronavirus Regulations: http://www.legislation.gov.uk/uksi/2020/350/regulation/8/made

But, hmmm

Yes, @CollegeofPolice guidance says one thing

@TVP_Specials happily tweet about doing the other https://twitter.com/jbond/status/1252567151451541504

This. https://twitter.com/quietscientist/status/1252568811913596928