I don't think Russia would go to war against Britain in the 20th century unless the Germans were on their side and the French neutral, or vice versa.
You mean modern Russia or like the USSR?
Depends when your POD is. Russia must get through its conquests in Central Asia first, so at the earliest after mid 19th century.
The thread is about when it
could be done, not if it
would
The Russians invading India is a recurring scare in British politics from the Crimean War onward. It really isn't possible to move a large army from Russia into India for much of the time. Its possible in the 15th century when armies had little or no supplies. Its possible today but 18th or 19th century army, with its artillery and wagons isn't making it. In World War One, an army couldn't go much beyond 100 kilometers from a railhead
Certainly by the 1970s when the Soviets invaded Afghanistan, it would have been physically possible to move an army into India. Trucks replacing rails and horses allow for an army to move over the terrain
Numerous scenarios could lead to war between Russia and Britain- an Anglo-French war over Africa, Britain intervening in the Russo-Japanese War or a Russian Empire that survives World War One might have a falling out with Britain. A Russia that pursued the Bjorko Alliance might have done it as well. An adventuress Soviet Union might do it while Hitler attacked Britain
I'm not really asking about the politics. I was more interested in the logistics- as when it would be physically possible. The border doesn't really change until the break up of the Soviet Union and the British leave india. Assume it doesn't change