Welcome to the bizarre world that is the U-U-U bunfight

, in the blue corner we have the unfortunately named yousuck Alliance I mean the USUK Alliance and in the red corner we have the USSR.
The rules are simple operation Barbarossa style invasion of the Soviet Union.
The ground work is rather similar by fair means or foul the USUK must secure the compliance, willing or otherwise of Poland, Hungary, Romania as launching off points for their assault. Now this is actually not all that challenging it just required extensive bribery something that the USUK are eminently to do as they have control of global trade...why they would bother already having control of global trade we shall skirt around.
In addition it may be possible to launch subsidiary assaults via other points with the relative compliance of hitherto neutral countries. Now again securing the compliance of countries like Turkey, Persia, Afghanistan or even Finland should be relatively easy for the USUK coalition massive brbery plus judicious application of military force....
It is post set up we run into problems. The USUK forces are designed to protect control over global trade, the only commodity in the world worth a damn as all the rest are merely components of it. They have supremely strong naval forces. Relatively strong air forces and land forces that can be characterised as efficient....yes efficient not large, the kind of forces you have if you would like to transport them to somewhere and drop them on the heads of someone trying to muscle in on your special interests but not really the kind of forces for a massive ground war in Europe let alone Eurasia.
To effect the invasion of the USSR would require massive changes to the spending regimes of the USUK and this distortion given that their priority is the control of global trade is chancy especially as when you already have control over global trade more territory grants you nothing but more headaches.
The Second World War was still fairly close to the kind of war that the USUK system was designed for. Fairly large mechanised armies were raised with some ninety divisions each by the US and the British Empire but essentially the bulk of logistics was handled by sea transport. To invade the Eurasian interior would require far greater allocation of resources to land transport for obvious reasons and the resources for that would have to come from somewhere. Either the home front and that would be a hard sell as going to war to make your citizens poorer is rarely popular in the first place or from naval forces as sea borne logistics would still be needed in the same amount and given that the defence of global trade rest precisely on total naval supremacy can you imagine how that will go down with the admirals?
So yes in theory the USUK could raise some 120 divisions for the assault and yes they would unlike the German plus allies of Barbarossa be largely if not entirely motorised and yes they would likely have more extensive air forces in support and yes it ought to be possible to provide more extensive logistics support to them thus avoiding the need for "special measures" against local civilians with all the complications that entailed. However none of that would be available in 1941 as the priority of the USUK was always maritime not land power.
When you already have everything it makes far more sense to hold on to what you have rather than grab for more. The downside of this, if it can be considered a downside, is that you then do not allocate resources to the conquest of large industrial states in the Eurasian interior.
Which means the USSR is going to be a hell of a lot stronger when attacked in 1942 onwards than it was when the Germans went in in 1941. Back then the USSR had vitally weakened itself in all kinds of areas not merely assassinating its own officer corps but uprooting its fixed fortifications to the new frontiers before the Red Army was rebuilt to the point it could handle a mobile defence.
This then is the perverse point of the exercise. The very strengths that make the USUK Alliance vastly more dangerous in terms of capacity to attack the USSR if they so chose to spend their resources thusly (which for obvious reasons they largely did not in OTL) actually preclude the desire to do so thus weakening their commitment to the kind of life and death struggle needed to overcome the USSR on its home soil. Unlike Germany the USSR simply cannot reach the USUK on home soil so you cannot sell your soldiers on an existential threat. Unlike Germany the USUK holds the whip hand when trading for Soviet resources, they have far more of what the Soviets want than vice versa which again was not the case for the Germans who simply could not afford to pay in any kind of long term trade.
Essentially yes in theory the USUK might be able to just about beat the USSR in its home soil but the odds are that the cost would be enormously prohibitive and would ruin the USUK economies to the point that any 'gains' would in fact be negative. Which means that at any given point in the conflict the USUK are far more likely to simply pack their bags and sail home...because they can.