I was on sky princess transatlantic and was popped as a close contact. they put me on dolphin deck in a covid positive quarantine room, even though i tested negative and never had a positive test. they refused to retest me, though i only had the one negative test about 5 days before disembarkation. i had an emed test in my luggage and self-tested negative, but that didn't do any good. my mom and daughter got tested the day before disembarkation in the vista lounge, but they wouldn't re-test me, kept me in quarantine, and gave me disembarkation instructions for covid positive folks. it was really awful. i had to get my own test at heathrow, after i learned how to sign up for the test via an app i had to download at the airport, and there were very few spots available for a covid test. waited about 4 hours for the test to be administered and 45 minutes afterward for results before i could go through security. all while i was separated from my family. I am super annoyed. so good luck to you. you can get a test at heathrow but i would advise finding out who does it and signing up for a spot the day before.