There is no doubt that the USA saved the rest of the world, let alone Britain, in WW2. Britain was broke by mid 1941 and could not afford to buy any weapons, food or other resources. Without Lend Lease, even Churchill would have been forced to make peace with Germany. You can’t fight a war if you have no weapons.
Lend Lease made US resources available to the UK. In 1941 US destroyers were escorting convoys in the Atlantic and attacking German U-boats. IIRC a US destroyer was sunk by a U-boat. My WAG is that Roosevelt would rather pay Britain to fight the war than actually get involved. I reckon he thought that the Japanese would back down and the US would watch from the sidelines as the UK & USSR ground down Germany between them.
Japan, by attacking instead of backing down, got the US into the war and made the war shorter. Shorter? You ask. How? Easy, the USA turned itself from a peacetime to a wartime economy, putting all of its resources into fighting a war rather than just some, and making US manpower available. As an active ally and the largest, the US could now direct the war rather than let Britain do it its own way. As an ally, the US had the industrial resources to build liberty ships, to supply tanks, trucks, transport (especially) and combat aircraft to all of its allies. It also had the scientific resources to develop the later, more complex radars (major radar inventions were invented in Britain) and to build the A-bomb (with a lot of British physicists). Britain could do all of these, but not together and on the scale to fight a war.
The Battle of the Atlantic was the key campaign but it was badly managed by the UK. Too many long range aircraft went to Bomber Command, where they dropped bombs on Germany, usually missing anything worth bombing, not enough to Coastal Command where, even if the aircraft never saw a U-boat, being seen by a U-boat kept it (the U-boat) underwater and unable to attack a convoy. Generally speaking, not enough resources were put into winning the Battle of the Atlantic, which is why it took until 1943. For all Churchill’s comments, he bears responsibility for not overriding the “bomber barons” and ensuring resources were correctly allocated.
And a final point. Convoys were vital. Unescorted ships are easy targets. Sticking them in a convoy, they act as live bait, drawing the U-boats towards the escorts who are the best equipped ships for dealing with them. Convoys sank U-boats. U-boats sank unescorted ships.