For Christmas (never Xmas) fares, I always wait and book in the October/November sales when the best deals are to be had. Currently exLON California is running at £2300-£2500 per ticket, but a sale fare of £1900 is more typical if purchased in the October/November sale.
exAMS is around €2400 rtn right now for LAX; I’m sure there are cheaper starting points.
BA will be flying its A380 to LAX by then. And don’t forget BA’s 777 San Diego service in case you fancy a quieter airport and visiting the Queen Mary in Long Beach.
Oh, and there is BA award availability out on the 19th, back on the 8th. I only checked Club World, not F cabins and didn’t check SFO, SAN or partners.