Thanks for the excellent guide.
I put it here since I tried to follow the sticky but I failed - so this could be interesting for other members as well. Feel free to move to a better place and sorry if this has been already answered in other threads.

I have about 28K household Avios that I would like to use for UuA this year or next year. Before FT times I made the mistake of collecting points with two alliances, and my intra-European business destinations make it unlikely that I will do much BA in the future, but I have the occasional USA trips once or twice a year where BA could be a one-time option.

The reservation will be via a travel agent (so I want to check availability first), and potential itineraries are BUD-LHR-SFO-LHR-SFO or BUD-LHR/LGW-MCO-LHR/LGW-BUD on ET/WT. May be PHX or LAS, but less likely. I am exploring options on BA’s Avios upgrade bookings and facing problems.

The trick mentioned by KARFA in the thread with stopovers worked for me as well in some extent (i.e. less mysterious errors than using the without stopover option and wider selection of flights), but I still experience weird issues with the leg selection. I read in the sticky that individual legs can be selected for UuA. My preference is to upgrade the Transatlantic leg(s) to WT+, but in many cases the site seems to force me to also upgrade the BUD-LHR or LHR-BUD sector to Club Europe or to upgrade to WT+ for both transatlantic legs. Sometimes there is no other column for ET/WT to select a flight via the radio button on that leg, sometimes the column is there but listed as not available. In some cases for the same dates I was able to select ET (I played around with Inbound or Outbound only redemptions on the first screen), but when I tick both directions it seems that ET/WT are not listed though I do not think those flights are full (I tried with March, April and May).

Is there a reliable way to cherry-pick the sectors as I like (assuming there is availability of course)? My understanding is that the 28K should be enough to upgrade to WT+:
LGW-MCO-LGW sectors off-peak or
One leg MCO (and two CE legs off peak if I want) or
One leg SFO (and one CE leg off-peak if I want)
Anything else would require more Avios than I have - so this is a blocker for me.