How is this realistically supposed to work?
Day 2 test may be taken anytime on Day 0 (arrival date) or Day 1 or Day 2.
So someone arrives on Day 0 can go about freely on that day. Likewise on Day 1. And finally on Day 2, travel to a place to do PCR test (likely using public transport) then back to home or hotel, before starting self isolation? Don't think Boris or Sajid have thought through the execution and practical realities.
Unless all incoming travellers are compelled to do on-site testing at the arrival airport (imagine the queues at numerous UK airports, and the resultant logistics nightmare!) and put into isolation until results come back negative, this isn't going to achieve the intended outcomes of isolating potential positive passengers.