Re: a quote from American journalist David Fairbanks White, …
It's all interconnected.
While UK is in the war, then
1) Britain is a base for operations against German war production which Soviets aren't in a position to attack
2) Royal Navy maintains blockade of German so they don't have access to tungsten, copper, rubber and other things useful for making tanks and ammunition
3) USSR gets some of the Lend Lease via Persian corridor
4) British supplies to Soviets include 5,000 tanks (inc Canadian Valentine production), 1400 radar sets, 1 battleship, 4000 or so aircraft, 13000 tons copper, 114000 tons rubber, £13 million worth of machine tools etc
On top of which are supplies from USA to USSR sent via Atlantic. So keeping the Atlantic open affects the war to the East.