Britain also was about bankrupt in 1940, and rationing was already on before the BoB, - something the Germans started in 1943 (?)
Indeed Britain was the commonwealth, - for instance the pre-payment for the lend-lease deal with the USA was picked up by a U.S. cruiser, - from S-Africa.
Germany also had a good back-up in the time of the BoB, a deal with the USSR (oil and other raw materials), parters and subjects like Hungary, Austria, Romania, Italy, heck, even Spain for a bit, as well as the illgotten gains from the recently occupied countries of W-Europe and Poland before.
So, basically you could say that Dowding had every reason to worry.