"01 UuA on BA only flights, booked through a BA sales channel
You can upgrade from economy to premium economy on all published fares except Q, O, and G. You can upgrade any World Traveller Plus or Club World fare to the next available cabin. One-way UuAs are possible at any time as is cherry picking individual sectors for upgrade. "
Maybe I am misreading the above advice, but when I tried to upgrade domestic legs of INV - AUH rtn from Y to J I was told the whole ticket had to be upgraded using Avios, I could not cherry pick. This could not happen because there was no avios availability on the LHR - AUH sectors. The overall ticket has been booked in W.