Upgrading Using Avios (UuA) & Gold Upgrade Vouchers (GUF)
A quick note on doing UuAs is that you can do these for exEU itineraries (or you can apply a GUF if you have any). The following restrictions for UuAs apply though:

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

The rules have previously said that the TA has to issue the ticket in the country of your BAEC membership as well. However, apparently this is not enforced consistently so hopefully this shouldn't be an issue. See these two posts for more information on that point https://www.flyertalk.com/forum/29280847-post33.html & https://www.flyertalk.com/forum/29282377-post38.html

A BA Holiday booking (flights & hotel) is UuA'able and treated as a TA booking, so a booking including non BA flights could not be UuA'ed. Also a BA Holiday booking which was made using a TA can not be UuA'ed.

Note that even if booked direct with BA on a 125-... ticket you can't UuA BA marketed and AA operated flight. The BA flight must be both BA marketed and operated.

There is more information on UuA'ing TA bookings here.