Originally Posted by
corporate-wage-slave
Yes you are correct, but with so much time in LHR I would personally do the test in the UK, not least to minimise the risk of arriving positive in the USA.
Whereas since the poster is apparently based in SWitzerland, I would do the test in ZRH (or maybe even nearer home if possible) and reduce the possibility of getting stuck in a hotel near LHR for a week if they test positive