There was, and to some extent still is a practice in Scotland of a son (usually the eldest) being given his Mother's maiden name as a middle name. There was no attempt to turn it into a double-barrelled surname but he was often referred to by the middle name as if his given forename didn't exist. I knew a Bob Davidson who was never called anything else but Miller by his mother, that having been her maiden name. Another friend was James Hayworth Donaldson and was known by everyone as Hayworth until he rebelled in his mid-twenties and became plain Jimmy. This could have been a mechanism whereby Scots (at least) surnames changed into forenames. I can think of Bruce, Grant, Craig, Douglas, Forbes, Farquhar, Fraser and Graham for starters.