I don’t think there is a hurry to interdict. The important issue is complete Naval / Air Dominance over the Atlantic.
Here is how I see it,
I buy 2 subs, 1 carrier. (or substitute 1 sub for a destroyer)
I send 2 subs to 106. Baring bad dice, the destroyer / transport will be destroyed and there will be 1 or 2 subs left. Unless the UK player sends the destroyer from 109 after 106, convoy damage of 3 should be given.
All the sea zones but 111 and 109 are cleared off. If the UK player joins it’s fleet in 109 on his turn (2 destroyers, 1 battleship and 91 cruiser + 3 planes, or 4 planes if leave a loner in scotland + whatever they build) then I will have to counter on 109.
3 subs from 112 (original 124 plus purchases)
Likely 1 sub from 110, although not sure
1 Battleship
1 Cruiser
1 Carrier
1 Bomber
All my tac / Fighers, including those from S Italy (because of carrier)
The fleet can come from 111 / 119 (from 112) or from 110, so this thing is hard to block.
So if the UK joins up in 109, I’m perfectly fine with that!
So then it’s clear that the UK player won’t consolidate in 109. So he’lll need to put the 2 destroyers, the battelship and the cruiser somewhere else, but where? Cruiser is probably best suited to go help in the Med (e.g., block 92).
Zone 124 is a bad idea, too many planes (bomber, carrier planes + planes landing on carrier) / subs / surface fleet can reach
Zone 123 may look safer, because less planes can reach, but all subs from 112 + surface fleet is enough to do the job. Add to this the subs from 106 and this is a bad idea.
119 is not safe either, can’t easily be blocked
118 may be the best option, but still need a block in 119 and 109… Again, this is in reach of bomber, and carrier planes.
So I think the UK Navy will be finished off at this point with minimal cost to the German (at this point, your surface fleet can hang out in 109 to hold it for good, forget about the destroyers). The German fleet needs to be built anyway to deal with the US.
Then start building subs in Normandy to send to 91 / UK Convoy zones. This forces the US navy to spend at least some in the Atlantic. Subs are cheap, expandable and hard to kill. They can help build a formidable defence in Gibraltar, or if appropriate, a force to counterattack sea zone 91 (along with planes).
I have not determined when the US enters the war. I am praying for the UK to declare war on Japan. Sure thing, my Germany won’t try to invade the UK (although I might if opportunity present itselfs, e.g., lots of builds off Canada), I will isolate it and overwhelm it with impossible odds to defy the German fleet and air force.
Meanwhile, Italy better kick butt in Africa to further reduce UK’s relevance in the game.