GPU confusing as hell!
 

Trying to book SEA-LHR on W fare Aug 2 to 13. System shows no R avail. But if I book the same ticket in Y, B or M, the system seems to show there is R avail in all flights??

So if I go ahead and book this, does UA still release R way before flight, or I am likely to get stuck the day of the flight to find out?

This is during the Olympics as you know, and fares are high and no seats are released for R availability. Given the number of business travelers going to the games for corporate entertainment there MIGHT not be many upgrades given during this time. On the other hand, regular business travelers are likely to avoid London during this time so you might have a chance.

Not sure how you are seeing R availability by booking in Y or B or M. They are unrelated. Perhaps you are seeing the Premier upgrade that applies to DOMESTIC flights booked in those classes (m only for Platinum and 1K). No international flights to LHR are showing R availability on those dates.

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.)

I agree; I don't see any R availability on flights for your outbound flight in any fare class, even on the domestic leg (I didn't check the return). Not what you are seeing. It does show that some fares are Premier Instant Upgrade Fares, but there's still no availability.