Thank you, Der Teddy Bear, for your very informative and thoughtful reply. For the record, I don't have a problem with my sub being sunk, as we all know that the odds are definitely stacked against the U-boat.
I've been doing some research into the fates of U-boats and trying to come up with predictor variables. According to my notes, U-27 was sunk by British destroyers HMS Fortune and HMS Forester. This is not to say that the other 8 destroyers didn't participate, just that they weren't credited for the sinking.
From what I have read, there were a total of 16 Tribal Class Destroyers in the Royal Navy. I thought it was unusual to see three of them in such close proximity, but I suppose this isn't outside the realm of possibility.
To clarify: The initial pair of destroyers that I encountered consisted of one Tribal and one C-Class. The other three destroyers (and one other escort I didn't identify before sinking) came on station later. This was another aspect that I thought was odd, but apparently not totally improbable (just unlucky on my part

).
The other question you answered (the one I have been intending to ask) was about the efficacy of attempting to sink escorts - whether doing so would eliminate that ship from the enemy's roster so that it couldn't be encountered again (potentially a good early war strategy, prior to the Lend-Lease act). I figured that this would be a nightmare to code and didn't expect it to be a very likely possibility. Still, it's good to have the confirmation that sinking escorts would only have limited short-term benefits.
I have been enjoying NYGM TW 2.2 immensely and will no doubt continue to do so. Thank you and your development team for putting together such an enjoyable mod and for your willingness to be so responsive to questions.