Looks like it was the BA28, HKG-LHR service...diverting due to weather at LHR.
Can they take off with a full fuel load? Possibly not -I haven't got the books in front of me, but normal procedure in circumstances like this ( diversion to an airport relatively near final destination) is to stick perhaps a max of 40ish tonnes of fuel on, not fill it up to full ( around 170 tonnes).