The only way to buy access to BA lounges is to purchase Qantas Club.
More details can be found here:
http://www.qantas.com.au/info/flying/qantasClub/index
You can join using the following link:
http://www.qantas.com.au/fflyer/do/d...=setFormValues
In a nutshell, QF Club gives access to AA, BA and QF lounges when flying AA, BA and QF.