I'm not sure that BA offers priority baggage anywhere on ba.com, either by class of travel or by BAEC level. I have looked just now at their pages on travelling CW and F and it doesn't mention priority baggage anywhere that I can see. Also, on the OW site, where priority baggage is an explicit OW privilege to Emeralds and Sapphires, it explicitly mentions that BA does not participate in this benefit.

When checking a bag, in any class, I usually have a priority tag affixed to my bag - but, as mentioned elsewhere, this appears to have no effect ex-LHR.