PNR. I believe it stands for personal number reference...but I could be making that up but it’s definitely stands for booking reference. Ticket numbers are 125 as you said.
PNR stands for Passenger Name Record. Strictly speaking, the alphanumeric reference is called a PNR Locator, but booking reference or PNR is more user friendly for most people
if this is booked with IB on an IB flight number, it will not be on a (BA) 125 ticket, it will be an IB ticket starting 075.