United doesn't operate a nonstop flight SEA-LHR. Married-segment availability display can cause some quirks in how R availability is shown on united.com, but not how it is actually made available for use on the transatlantic segment.

Try looking for R availability on the exact nonstop flights to LHR you're looking at. Gateways to LHR for United include SFO, LAX, IAH, EWR, ORD, and IAD.

If a sample booking for, say, IAD-LHR shows "R1", there is one seat available for confirmable upgrade, regardless of what is showed on united.com when you look at a sample booking for SEA-IAD-LHR. (Specifically, a sample booking for SEA-IAD-LHR may well show "R0" since SEA-IAD is often R0.)