Ha, I think just ran into that same convoy last night (except on Sept.11/39, just off the coast of Spain).
I was ordered to sink merchants, but I figured the loss of the battleship Q.E. would hurt Churchill more than the two (at best) tankers I'd be likely to hit on my first attack.
Took 3 hrs (game time) to reach a firing position. Fired all 4 forward tubes at the Q.E. with 2 duds, 1 miss, and a single hit to the stern. Turned the boat and gave her both stern tubes for another dud and another miss.
1 hit didn't even slow her down.
P.S. Under the rules of engagement at the time, any merchant under escort of an enemy warship was considered to be an enemy merchant.