Thought I’d throw in some numbers here:
In WWI, 5 of the eleven British battleships sunk by enemy action were sunk by subs, 3 others by torpedoes from ship or shore, and 3 from mines. None were sunk by gunfire. In WWII, the Brits only lost 6 battleships in combat; 2 to submarine, 2 to air attack, one by surface gunfire, 1 to explosives placed by frogmen.
British cruisers and battle cruisers – In WWI, one might say the results were skewed somewhat by the battle of Jutland, but here goes: 15 losses in combat, 6 by submarine, 6 by gunfire at Jutland, 2 by surface gunfire in another action, one by mine. WWII saw 23 cruisers sunk by enemy action; 9 sunk by submarine, 10 sunk by aircraft, 1 sunk by surface gunfire, 1 sunk by mine, 2 sunk by torpedo boats.
So submarines did a very good job of sinking naval warships even though that was not their strategic goal.
Rickjay, destroyers would fare no better than any other surface ships if bottled up in ports. As far as aircraft, you are correct, but note that airborne radar that could detect surfaced submarines didn’t exist at the beginning of the war. So it’s something of a crapshoot to guess when it might have been developed under different conditions. In historical terms, U-boats were not seriously menaced by either surface forces or aircraft until early 1943.
And as to the OP, would Hitler have insisted on an invasion if Germany had sufficient U-boats to successfully blockade merchant shipping heading for England? Would Churchill or the British have offered terms had the blockade been successful? Another unanswerable.