Rang the GS line who were not any help - they will not do anything until your flight is actually cancelled.
What is the best course of action? I hope/assume UA will rebook me on the next flight out of LHR, but how procative do I need to be?
Do I have any options, other than going by boat??
My experience this morning is that they were happy to rebook me without it being cancelled but it is a TATL flight if that makes any difference. Of course it could all have changed in the two or so hours since I spoke to them.
UPDATE: Flight got cancelled for tomorrow 1035am 955 LHR-SFO, now rebooked on the earliest flight available which is Monday.