Not done SEA but echo what others have said about avoiding the early flight and early arrival into LHR, based on other routes that I have flown. Those flight times seem almost perfectly designed to induce the worst jet lag ever.

It was one of the reasons I hated the re-timed BKK flight pre pandemic and also why some of the CX early departures to HKG always leave me feeling whacked. I have in the past tried pushing through the night before the flight, in attempt to induce my body to sleep immediately on settling into my seat, but with limited success. And the last time I did this I dribbled soup all over myself as I passed in and out of consciousness.