It's in this thread:

https://www.flyertalk.com/forum/brit...agreememt.html

If you are booked via BA then you won't get seat selection, until OLCI, when you can buy seats at that point. Or if you are a tight wadge you can ask the crew on boarding. The slight issue on 145s is trim, so it may be a case of accepting what you are given for take off and landing. Unfortunately Loganair isn't oneworld unfortunately, so the usual benefits don't transfer over.