[Furia]

Well regarding the Aircraft not being a threat for sub until the 43 is not true.

Aircraft posed a threat to the U-Boats from the begining of the war.

This boats were lost due aircraft operations of different sort.

U-55 Sunk on January 1940
U-64 Sunk April 1940
U-26 Sunk June 1940

In 1941 a total of 5 U-Boat were lost related to aircraft attack one sort or another.

While there was no specialized ASW weapons on most aircraft on the early days it is more than clear that if a sub was spotted, it was attacked.
There are records of countless crash dives due the presence and attack from aircrafts even on the early days.

Depth charges (Mk VII) were tested and installed on the Coastal Command Sunderlands by spring 1940.

While it is unlikely that Spitfires would have detected a sub due they usually were flying high, a number of other planes types were a real threat for the U-boats.