This threat is foolish and idiotic and unpleasant. Here's why. 1/
2/ Britain has few military assets relevant for this task. Its forces are better configured (reasonably) for maritime defence.
2. The fall of Eastern Europe to Russia would bring Russian troops closer to Britain. This wouldn't help British security so it's an empty threat.
3. Poles remember Britain didn't live up to its commitments in 1939.
