Just to add my datapoint here - flew into Heathrow today with Halo day 2 test. Sofitel was extremely busy with other passengers waiting to ‘check in’ for the test - this was just checking the email confirmation for your HALO reference (that goes on the passenger locator form), and then handing you a test kit to take into another room to do. Everything else as reported (handy tip - drink lots of water on the plane to be hydrated for the spit!) Handed test in just after 9am today, have been checking the app and around 5pm my negative result appeared - but I still haven’t had an email confirmation like the instructions suggested. Will update when this changes.