The question is how much did lend lease add to Britain's fighting power between the fall of France and the start of Barbarossa.
In this period of time that can't be too relient on lend-lease vehicles, considering they sent ship loads of vehicles and tanks to the Soviet Union.
It becomes harder for Britain, but by no means impossible nor a reason to give up. Germany still can't invade, Britain has a harder time in Africa and a more impossible task on the continent.
British industry is safe from German troops and bombers and so just keeps building its own things. Takes a lot longer to do everything of course, but eventually the US has to get involved and without lend-lease
the USA has more of its own tanks anyway, making it easier for them? 