United is very stingy with premium cabin award space.

Saver space on SFO-LHR is almost non-existent. I can save you some time because I just checked the entire UA calendar and there are zero SFO->LHR saver flights, and zero LAX->LHR saver flights. This will almost certainly not change for May 2023.

London-NA is super hot, lots of pent up demand in both directions.

Not sure exactly what your airport is, but I think your best bet would be some kind of routing either on LH/LX from SFO/LAX/SEA to MUC/FRA/ZRH, and then a hop to London. Those carriers are more generous with award space than UA.

Alternatively, go through YVR and then fly on Air Canada. They still release some far out award space, albeit less than in the past.

I'm not sure about all this with 2pax though. I would just use the miles to visit Asia on SQ to get the best value, and to have much better opportunities to grab 2 seats booking far out. Obviously that is not what you asked for though.