Umbrellas, under BA's cabin baggage policy, are supposed to be "small and foldable" and yours appears to be somewhat more than that. I would say it's unlikely you would be stopped with it, since it presumably can go on the top of the overhead luggage bins, and therefore not an issue. Note that umbrellas must be one of the most frequent items to be forgotten about and left onboard at arrival. If it has a sharp point at the end then security may have an issue.