It is unlikely you would be able to process a UuA for a TA booking through ba.com - I am actually surprised you got it to produce a quote at all. You would have to call to UuA a TA booking usually.

Typically UuA doesn't work via ba.com unless you have a very simple booking (such as a LHR-JFK WTP return), and have no complicating factors such as it being a TA booking.