I have a question regarding exactly this topic. Once I succeeded my skill test and then my base training, my license was issued the very next day. (maybe even just after the skill test, I forgot)
So, what would prevent me from operating as a first officer on that day ? When I finished my line training and succeeded the line check, nothing on my license changed.
This means that the airline could roster somebody halfway through line training on a normal line flight, chances are everything would go well, and even if there was a CAA check, the licence would be valid. They would not be able to see problem, would they ?