The only way AI Russia can do what the original poster experiences is if a human UK player is inexperienced. The human UK player must be prepared to strike at the German homeland as soon as German forces are deep into Russia (but not at the point of victory). This is the only way to stop the Russian juggernaught from claiming nearly of all of Western Europe, possibly even France.
In the above please note that I say to "...strike at the German homeland...." Any thing short of this will always result in a Russian victory. By the time the Germans are deep into Russia you must be prepared for, and do as, follows:
1. Have two complete armies (25 divisions each, mostly fast).
2. Use one army to strike at Denmark and around Hamburg. The objective is to rapidly free Copenhagen and open the Baltic.
3. Once the Baltic is open invade at Stettin with your second army. This army must simultaneously free Berlin and create a UK "corridor" stretching from north-to-south to have a border to prevent westward Russian encroachment. Since this corridor can be achieved with only 3 of your five corps you use the two spares to drive into Poland and take it (do not liberate it).
4. Defeat Germany.
The above leaves you with a situation where the Russians will then only have the option to drive through Romania. Since you have let the Germans and Russians bloody each other (probably for no less than one year) the Russians will only have enough time to liberate Romania. You, however, have plently of time to conquer Italy (w/o Germany it falls very fast), Hungary and Bulgaria. Then take your sweet-assed time preparing to take out the Russians with your own version of Barbarossa, though for this latter operation I suggest jumping off as soon as your forces can form a line.
Yes, Russia is very overpowered by most definitions of overpowered, but if you have followed the evolution of hoi3 you would know that every expansion included "buffs" and rule changes to give Russia every chance to win. This was to prevent the constant littany of complaints, all valid, about the German AI's ability to absolutely steamroll Russia.
The only downside to this is that playing Russia as a human has become very dull. Just for fun I recently started a Russia game. It is October, 1939, and I am already defeating Finland and Romania. I have completed 3 CVLs (with CAGs), 4 new BBs (with good guns, but bad armor (who needs armor when you have CVLs?)), and by about March of 1940 I will own Romania and Bulgaria. Finland, Greece and Yugslavia will all be Russian puppets. When Italy tries to take Greece (and it will) the MR-Pact will freeze Germany and I will defeat Italy with ease, thus having wartime laws for the rest of the game. I doubt Germany will ever execute Barbarossa, and even if it does it is at a laughable disadvantage.
Oh, yes, Russia is very overpowered. This is an accurate portrayal of what a real-life Russia under "proper" management could accomplish, both past and present. Luckily, for the rest of the world, Russia has yet to master proper management