Yes this is fine. An exEU is just a UuA'able as an exUK booking. Bear in mind it must have been booked with BA, or of it os a TA booking is must contain BA flights only.
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
TA has to issue the ticket in the country of your BAEC membership as well.
You will pay the extra avios and the additional carrier surcharge to do the UuA.