Anyone can have a test but that is not the same as saying you can have it exactly when you want it If you want it when you arrive.
But suppose we tested everyone at the airport, Test results take a day couple of days to come back so you'd have to quarantine until the results come through. And whilst a positive test would confirm you must quarantine a negative test would prove nothing unless you had a follow up test maybe 4-5 days later, so it may be safest to quarantine until then