It seems with 6 subs on the board, you could keep 2 off Canada and 4 off London. That means you would subtract 11 IPCs from the UK until the US entered the war, if UK does not build a DD in Canada.
You could keep London at around 18 IPCs until the US enters the war. However, I don’t think Germany can spare the money on sea units, since I believe they need to help Japan gather victory cities or face a long term loss of game. I also suspect that the US will have 2-3 DDs in their war fleet able to clear two of the convoy zones for UK.
Personally I would make air units over sea units, as you can then threaten fleets and land units especially if you need a quick extra “push” against Moscow early on.
You would discourage a navy build by UK until the US could arrive on scene.
In some Europe games, I did produce a couple of subs early on, but not in any global games. I found their life span to be brief unless they hid behind Denmark.