Jerrystinger,
The 744 from LAX chose to continue as maintainance was available upon arrival in LHR. In this case it would have made no sense to continue to Sao Paulo, only to have possibly had the company fly an engineer out to Brazil to sort the problem out there, plus the cost of delaying the flight by a substantial amount. In this case I would imagine it is BA policy to return the aircraft to LHR to sort the problem out, minimise the knock-on effect and minimise the disruption to passengers.