The BA site redirects you to the Latam site when you ask about luggage. It does say those details there, but it is far from clear, so it's nice to have that confirmed thanks.
I presume that is in Economy? I.e. you get 'full fat' Economy Plus fare if you book with Avios.
Cabin baggage allowed on aircraft | LATAM Airlines
In any event it looks like when connecting from a BA flight, my cabin luggage will be my BA personal item and my BA Hand luggage will have to go in the hold.
So no 'hand luggage only' trips if you have more than a few kilos of stuff to take.