Why American purchases from neutrals were not Britain's main supply route

Did the United States commonly buy food or strategic materials from neutrals and then pass them to Britain through Lend-Lease?

Not normally. Britain already drew many basic supplies, especially food, from Canada and other Commonwealth mutual-aid sources.

The United States did not normally need to buy neutral raw materials or food simply to pass them to Britain. British food supply, for example, relied heavily on Canada. Those flows belonged to the Commonwealth mutual-aid system that complemented American Lend-Lease. The exchange shows why the Allied supply network must be read as several connected arrangements rather than a single American pipeline.