Technically, if you want to leave the departure lounge without taking a flight, someone from the airline is supposed to escort you out and BAA is in the process of segregating arriving and departing passengers so it's going to get harder to do this anyway.
Is it possible for you to get access to the BA Arrivals lounge?
BA's website says that its Gold members can access the arrivals lounge at any time
Gold members can enjoy British Airways Lounge Access anytime, regardless of the airline flown that day. Gold members may also relax and revitalise at the Arrivals Lounges at London's Heathrow and Gatwick airports, plus Johannesburg International Airport, regardless of the class of service they are flying.
but I don't know how this translates to oneworld partners.