Actually from memory they diverted to MAN after declaring a fuel emergency because they couldn't make LHR on 3.
Actually they did have enough fuel, if you read the report BA used there own non Boeing fuel balance procedure (basically using O/J pumps only and not turning off boost pumps) this only works up to a point and that point was Manchester after the investigation BA now use the correct Boeing procedure.
The crew had been using the override/jettison pumps to maintain fuel balance but these became ineffective towards the end of the flight. Thereafter, there was a reluctance to turn both main pumps off in a tank and a lack of confidence that this would be effective. There was ncreasng concern that they would not be able to keep the main tanks balanced and that some of the fuel might be unavailable.
A better understanding of the fuel system should have reassured the crew that fuel should have been avalable to all engines even with one tank empty. Nevertheless, the awareness of the apparent problem came at a time when the crew had made the decision to divert, had started the descent to Manchester and was therefore busy. If the crew had been n the habt of utlsng the manufacturer’s procedures for balancing fuel by only using the main pumps, it is possible that they would have become more confident with the procedure. Although the problem had not previously been encountered by other company pilots, the potential difficulties might have been foreseen by the operator. After the ncdent, the operator reverted to the manufacturer’s fuel handling procedures.