I arrive on 29th and the onsite tests at Heathrow are filling up fast for the day 0 - 2 fully vaccinated PCR test - not much availability at T5 - but plenty at T4 long stay car park. I have booked one to be safe as results guaranteed by 10pm next day and not having to self isolate beyond Tuesday is appealing. However, already submitted passenger locator form with antigen test - so clarity would be nice!