Could try emailing the etkt from AY site and see if the LA locator is included. I assume there's no option to choose seats and be passed through to the LA MMB? If neither of those, may have to call BA/LA and ask them. LA could look it up with the BA/125 tkt number.
The LAnnnnnnnxxxx code sounds like an NDC order number, seems odd that there'd be one of those for an Avios bkg, but maybe I'm just out of the loop...