Sorry, that doesn't wash. Stonehenge is between two-and-a-half and three millenia older than mainstream Christianity in Great Britain, yet after Stonehenge we don't see monumental architecture on such a scale anywhere. It is not proven that the Druids built Stonehenge, but even if they did, there is a big difference between "religion" and "civilisation".
So because no other monuments of that scale were built (or didn't last until present day (which means because no proof exists today means that it never existed under your claim?)) that means the pagans (or druids, or whatever you want to call them) died off (or their culture as you claim? I didn't realize they were a separate civilization) shortly after Stonehenge was built... I'm sorry, but that explanation just doesn't wash.