Forgive me if I'm a little confused as maybe I've not understood this correctly.

You're asking for an e-mail address for customer relations at BA where they will respond to you. Am I missing something - you say that they responded to your first e-mail to say the matter was being investigated and they would get in touch again. They have since done this, and also provided you with a phone number to contact them.

Surely in that case you could either phone them as they suggest, or contact them through the original e-mail address where they responded to you. Or if you prefer you can write to them at the mailing address shown on ba.com.