Sources suggested the issue could be the result of an incorrectly filed plan by a French airline,
Ah, the usual UK convenient excuse,
but regardless of nationality , this is an interesting comment since in Europe all flights plans are centralized and go through IFPS , checked and redistributed to the ATC centers by Eurocontrol . So if there was an initial filing error by the airline it should have been spotted at IFPS level. but assuming it was not, then every other ATC center receiving the FPL should have been affected, and if not why only the UK system .