The Air France lounge is not a One World lounge and so no access there
Generally you can use any lounge of a OW airline as long as you are departing from that airport on a flight both operated by and marketed by a OW airline ; so as long as it is accessible, generally you can use it
There are some exceptions ; e.g. Heathrow has rules that restrict access to lounges in terminal from which departing even if another terminal is accessible
If departing on BA, you should be able to use any OW lounge