If one is unsure of which terminals they will use at Heathrow, they can use the "Which Terminal" tool
here.
For
oneworld Sapphire or Emerald, BA at LHR follows the
oneworld protocols for admissions in all of its lounges AFAIK, excepting the Concorde Lounge - only for BA First longhaul travellers.
If one is somewhat pressed for time, the information desks can inform one which gates the scheduled flight is likely to depart from (not the exact gate until close to boarding time - but which area gate is likely to be used) so one can proceed to the Galleries lounge closest to the departure gate.