You answered your own question, you said the ‘aircraft was full due to a cancelled Virgin flight’. The crew would have worked out the fuel on the flight plan ZFW and were caught out by the extra pax. Yes there are normal processes that could have altered the crew to the change in zfw but that would take communication and a pro-active ground staff both of high are largely missing at the moment. First the pilots would have known about it would have been when the loading paperwork arrived at departure time. **** happens,