Where Britain obtained its wartime grain

Did the closure of Russian ports deprive Britain of a major source of grain?

No. Most British grain came more cheaply from Canada and the United States, while Russian exports went mainly to continental Europe.

A commenter suggests that the loss of Russian grain deliveries would have driven a major rise in British food prices. The answer replies that Britain obtained most of its grain from Canada and the United States. Their industrial-scale agriculture could supply it more cheaply, while Russian grain went mainly to continental Europe. Russia's own wartime supply crisis therefore differed from Britain's Atlantic import pattern, as did the effects of the closed Dardanelles on each ally.