If you book through the BA site it should only offer the BA codeshares and not the QR flight numbers, and it should be booked on a BA 125-... ticket. In that case you would get lounge access and seat selection as normal. The unbundling issues should only affect those who book on a QR ticket and QR flight numbers.
See this previous thread for some more information
https://www.flyertalk.com/forum/brit...ess-class.html