There is a summary for allowed/unallowed UuAs in the Tier Point thread, I have copied it below for reference.

Upgrading Using Avios (UUA)
Summary guide to bookings which can be upgraded with Avios:

• BA issued tickets (can include other carriers) ^
• TA issued (on BA stock) booking with only BA flights ^
• TA issued (on BA stock) booking with other carriers
• Tickets issued on non-BA stock