True but at their own peril
http://www.uboatarchive.net/BDUKTB30248.htm
Naval War Staff has sent a T/P to F.O. U/B, ordering that all attacks on French ships are to be avoided at all costs. This means that U-boats cannot operate in the Channel against troop transports. This operation (U 35 see also F.O. U/B West's War Log) was arranged because of Naval War Staff's orders that convoys could be attacked North of the Latitude of Brest even if the escort consisted of French Forces. It can be taken as certain that these transports sail at night; and at night the U-boat must be able to assume that a darkened ship is an enemy ship, even if in convoy. It is often impossible to establish the nationality of ships in convoy even by day and when flags are not being misused, but at night it is quite out of the question. I therefore informed Naval War Staff that, if this order is to stand, I cannot let the boats operate in the Channel