Because it's nothing to do with the British embassy, it's a Filipino regulation. If you didn't attend a CFO seminar, it would be the Philipinnes emmigration people at the airport who would refuse to let you board the plane if you didn't have a CFO sticker in your passport. If you did manage to board the plane, British immigration wouldn't refuse to let you enter the UK because you didn't have a CFO sticker in your passport.