@Lynxes:
As I said UK will gladly spend 8 IPCs to block several subs from being used against the invading fleet.
One DD can only block 1 one sub.
Ger attacks with 1 sub and air (66% shot UK dd misses), then Ger Non-Coms remaining subs to other szs and replaces new sub in Sz 5.
Now the Allies need a DD to take out the sub (if survived) in Sz 6 as well as the other subs that could have moved to Sz 3 or sz 7.
Just like the Allies aren’t going to move a fleet to Sz 6 if Ger can attack heavy, Germany isn’t going to move a bunch of subs so 1 DD and tons of Allied air can sink them. It is a bit of a cat and mouse game, but 1 Ger sub can force the UK to use at least TWO DDs (one to block sz 6, which gets killed, and the one needed to sink the Ger sub in Sz 6 if suvived the attack on the DD). The Allies then need additional dds to sink any remaining subs that scatter.
At somepoint the Allies need to go to Sz 6 fairly heavy, just to stop the negative trade of 1-2 dds for 1 sub. Thats a worst case +2 for Ger and potentially a best case +10 ipc for Ger for one turn. (depends on how long or how many rounds the constant trade of sub for dd lasts).