Short response: because Soviets weren't interested.

First, Allied soldiers inside Soviet Union could see (or hear) things, which would shed bad light on USSR - not even Gulag facilities, but just generally low quality of life (compared to Western standards). It was "manageable" (meaning, with coverage of secret police) with e.g. seamen in Murmansk, or aviation units (some RAF and USAAF units based in USSR late during war; and there was a famous Free French "Normandie-Neman" squadron), but ground troops? It would be a problem.

Second, Stalin had enough of own troops. What he needed, was supply - which led to Lend-Lease program.

I'm assuming you're asking about the British sending troops to help the Soviets in World War 2, as I very much doubt the British would want to help the Soviets in Afghanistan.

Before I launch into a response, I'll ask you provide a more specific timeline for when the British should be sending troops to help the Soviets.