Without the UK in the war the Germans divert no resources to North Africa, they don't need to invade Yugoslavia, and don't get involved in any Italian mess in Greece. Depending upon when the armistice in the west is, lets agree sometime before fall, 1940, the Germans can now devote their industrial effort to building what is needed for BARBAROSSA, not so many U-boats etc. They, of course, have more men and equipment not lost elsewhere as well as returned POWS (especially nice for aircrew). OTOH the Russians will not be getting any supplies from the UK or USA - if the USA does send anything it will be cash and carry not Lend-Lease.
No Yugoslavia and North Africa means the invasion can kick off sooner in the spring than OTL which helps some. If Stalin proceeds as OTL with no changes the early disasters are even worse. If Stalin makes better preparations, then it won't be the disaster it could be but they will be beaten back. Even with Stalin being more sensible the losses will be as bad as OTL in 1941, if he does not plan better the Germans have an excellent shot at Moscow. With all of German efforts concentrated in the east, as well as being open to trade and able to buy all sorts of key stuff on the open market, the Germans can put the effort in to fixing the Russian rail system to improve their logistics in a big way. The Luftwaffe does not have to put much effort in to home defense or other theaters, so the war in the air is much worse for the Russians. If the British are out the Finns may be less constrained, and between that and the better German situation Leningrad probably falls.
You can argue about LL but the reality is every meter of rail that came via LL was a meter that did not have to be made in the USSR and that steel could be used for something else. Same for trucks, radios, and on and on. Any and all food via LL could not be replaced by Russian food, similarly leather for boots, high octane avgas, and many other things. Even if all the factories that were moved OTl are moved here, you haven't added capacity which is what LL did functionally. Absent that, the Soviets will have a hard time preventing the Germans from getting to the A-A line.
Now ITTL this will NOT be kick the door in and the whole rotten structure falls but a bloody hard fought event. Saying that it is realistic to say that Russia alone against Germany, no LL, means they lose.