BA are keeping all the planes that where painted world colors in the world color scheme the rest are going to be painted Union Jack.
I do not know what percent are painted world but I do know that it is not half.
BA introduced the new scheme some years ago - it was however exteremely unpopular here in the United Kingdom. Although they have not admitted to it they have now abandoned the scheme in favour of the much more 'patriotic' revised Union Jack tail colours. As the existing 'world scheme' aircraft become due for re-painting they will recieve the Union Jack colours. Correct me if I am wrong - but I have this information from a source within BA.
Not only was the new scheme extremely unpopular in the UK, far more importantly was the fact that the scheme was unsuccessfull in the US. BA lost its brand image, marketing became very difficult, and Americans had liked the idea of flying a truely British airline.