Well, Canada and Australia had one of the largest militaries in the world after WW2. If the UK is really in danger I don't see why they wouldn't have the ability to produce enough to let the UK stay in the war (not enough for an ambitious D-day landing of course).
So 17 million Canadians and Australians are enough to compensate for the absence of 140 million Americans? Grasping at straws here! Not to mention that the distance Australia-UK is between 4 and 5 times longer than UK-Canada/US. So even IF the Australians could for some reason produce more than OTL - which they cant - it would take far longer to ship it to Britain which would basically be a waste of shipping space.
Why shouldn't the Anglo-Soviets just wait until 1944 and then drop Operation Vegetarian?
Firstly because without LL Britain will have capitulated/negotiated long before 1944 and second the fear of retaliation.
Given that the vast majority of LL deliveries to the Soviets took place in 1943-5, we can conclude that LL was probably not a pre-requisite for the victories at Stalingrad and particularly Moscow
True for Moscow, debatable to some degree for Stalingrad.
The key difference would then lie in the inability of the Soviets, via inadequate motorisation, to exploit any victories in 1943 onwards. Hence the conclusion that that Soviets are probably capable of winning the war, but it would be a much bloodier and slower victory at best.
Mostly false. The absence of LL food, aluminium (50% of wartime Soviet supply) , radios, power stations, gun powder, fuel, tanks, aircraft, motor vehicles ect ect would have put so much strain on the Soviet economy and military that stalemate would have been the best achievable option. Just holding the mid 1943 frontline would have taken everything the Soviet economy and military had, large offensive operations would have been out of the question. Especially in this TL where German industry would have been less damaged by Allied bombing and where the Germans could transfer additional troops to the Eastern Front.
For Britain, it requires astute diplomacy and industrial and military strategy to make effective use of limited resources. They need to keep Japan out, avoid wasting scarce resources on strategic bombing (the response to the Butt Report will be absolutely critical here) and focus entirely on supporting the Soviets via the Arctic route and, later, the Persian corridor. Difficult, certainly - particularly with the wild schemes of one WSC liable to run amok. It's arguably improbable, but it's not ASB.
How exaclty can the British make a greater effective use of limited resources than OTL? How exactly can Britain keep Japan out of the war if the Japanese decide to go south? What exactly are the British doing when they dont bomb Germany? How exactly are the British building up the transportation system in Iran without American rails and help? And what convoys are they sending via the Arctic if they need everything for themselves and cant spare nothing??
As noted before Britain HAS to spend every piece of gold/Dollar it has in the September 1939-April 1941 period because it NEEDS all those things from the US. No other nation can supply as much as quickly as the Americans. So the British are broke by April 1941. If LL doesnt kick in they will continue to get shipments until September/October for stuff they paid in advance and thats it. Thats means NO expansion of British industry in 1942 because there are no resources. That means severe food shortages in the winter of 1941/42 and constant famine thereafter, that means no LL aircraft, no LL tanks (50% of British OTL supply in 1942) no 8 million tons of additional shipping space built in 1942 by the Americans. The British loose Malta, they loose at El-Alamein and they loose Cairo. Hell they even might loose Gibraltar. Thousands of people starve to death every month, the Germans sink between 3-4 times more shipping space than the British can produce, even without a happy time in American waters. Should they still continue the war under these circumstances then there is the winter of 1942/43 which will be absolutely disastrous. Even if Britain doesnt surrender/compromise by late 1942 it will have effectively been knocked out of the war.
As for the Soviets, they get NOTHING from the British because they need everything for themselves. So 1941 plays out much like OTL in the East. There are some minor changes during 1942 though, the Soviets probably have to decide between Mars or Uranus because without LL they cant do both. The real changes start to kick in by 1943. The British are either out of the war or reduced to a nuisance. That means thousands of additional aircraft for the Luftwaffe, hundreds of additional tanks for the Army. German industry will produce more during 1943 than OTL. The Soviets on the other hand are short on food, short on resources, short on everything. So Soviet industry produces less than OTL. The Red Army is missing 150 000 US trucks and jeeps, 8000 LL tanks, 15 000 LL aircraft ect ect. Just keeping Soviet industry going and repelling all German attacks will require everything the Soviet state has. Most likely the Germans win at Kursk. Most likely the USSR experiences a famine in the winter of 1943/44. Even if all of these things dont happen, the Germans will be strong enough to repell all Soviet attacks resulting in a stalemate. A stalemate that will keep Soviet economy deteriorating, forcing them to abandon the war sooner or later.