Please accept my apologies in advance, but there's a terminology issue with several posts here.
BA domestic services have one class of service: UK Domestic.
BA sort haul flights have two classes of service: Euro Traveller and Club Europe.
BA mid and long haul flights have two, three or four classes of service: First, Club World, World Traveller Plus and World Traveller.
It is therefore not possible to be upgraded from World Traveller to Club Europe.