One individual who has often played a key role is the Private Secretary to the Monarch. Vernon Bogdanor devotes a whole chapter of his 1995 book "The Monarchy and the Constitution" to this office holder. He claims that "the office...has become crucial to the working of constitutional monarchy in Britain." It is a chapter worth reading!
The official description of the post on the UK Monarchy's website can be accessed here. The photograph accompanying this post is that of Lord Knollys - Private Secretary to Edward VII and George V.