Bear in mind that once fuel jettison has started it will be dumping approx 2T of fuel per minute. Normal reserve and diversion fuel for somewhere like LHR in good weather may be around 7T - 8T. Add on a few tons of contingency fuel would mean they would normally arrive at LHR with about 11T. If they had commenced fuel jettison within 5 minutes they would not have enough fuel to make LHR!
I am sure they would not have commenced fuel jettison, cancelled it and then elected to continue to LHR.