The SI has in fact now been published: https://www.legislation.gov.uk/uksi/...0200568_en.pdf

It covers the "Dublin Loophole" by stating that "a person who arrives in England from within the common travel area who has been outside the common travel area at any time in the period beginning with the 14th day before the date of their arrival in England must provide on the Passenger Locator Form their passenger information on their arrival"

The enforcement provisions are quite interesting and allow the use of force to return someone who should be self-isolating to the place where they should be in self-isolation.