BA have a (I think unpublished) benefit for their own Elite FFs that hold luggage can go up to 32kg, but this does NOT apply to other OW frequent flyers as discussed here at post 2. That poster is a BA employee and knows her stuff, so please don't take it out on her.

It's one piece in economy. You can pay extra during online checkin to boost the weight to 32kg; you may be lucky at the airport and not have to.

That's life - it is economy, after all (and I write as a some time economy flyer).