Why was England so worried about possible Russian control over Central Asia?
Short answer: Because it meant that British control over the area was not inevitable. Brittain was slowly moving north and east from their stronghold in southern India. With no other mayor European powers in the area, it seemed as if they would just move on as far as they could imagine. Then came the realization that far, far away, there was Russia slowly moving south-east. Far off yet, but they were there, and they were moving. And eventually, may be in 20 years, may be in 50, may be in 100, they would meet up.... So suddenly there was the realization that the land was not endless and eventually borders would have to be draw, either militarily or through negotiations or through some other means. So it would be best to make sure that when that time comes.... Not IF but WHEN... You would have the best starting position.