I’ll add a dose of realism here. The majority of passengers know nothing about tunnels or walkways, and would get confused at the very mention of trying to be clever, so the default policy is to send everyone to A to avoid problems later. However, if you are calm, friendly and explain that you know what you’re doing, they will always let you through regardless of which flight you’re connecting onto. It’s British pragmatism at its finest, and (apart from the limited opening hours post-covid) works remarkably well. And as so often at Heathrow, those who try to be arsey get put in their place.