Yes you need a valid passport to travel to the UK.
The flight time is about 15 hrs fastest travel time, so please plan accordingly to make it to all wedding events
I have found it best to travel on a Tuesday or Wednesday for the most affordable travel expenses and to travel out of San Francisco airport or Los Angeles airport.
There is 4 to 5 hotels around the wedding venue to stay at and the location is 20 mins from the city center.