Hi Henry,

The Hertfordshire version was collected by Lucy Broadwood in 1914.
However (IMO) the evidence for Somerset is overwhelming.
1) All versions that have an intact beginning are set in a seaport.
2) Where it is present Bridgwater is a place not a person or an estate.
3) Another ballad The Bristol Tragedy has wording very close to the opening stanzas of Bramble Briar.

And that's apart from the fact that nearly all English versions were collected in a 40-mile radius around Bristol where it is highly likely it was first printed.

Before I came across the Bristol Tragedy I was looking at the possibility it was an American ballad originally as there is a Bridgewater in most of the states on the eastern seaboard.

The confusion between Hertfordshire and Herefordshire has happened before in ballads, and could have been deliberate or accidental. Broadside printers often relocated ballads for obvious reasons, but sometimes the apprentice copying a ballad down in a hurry would make the mistake.