A book by Michael Reed titled: The Making of Britain , The Age of Exuberance 1550 - 1700 , contains a number of thoughts that seem important to place in this discussion. On pages 19 - 21, he discusses the Act of Union and a number of factors not previously presented. He states that the Act of Union took some time for the details to be applied. He notes that the first Welsh Justices of the Peace appear in 1541, and it was not until March of 1543 a complete list of Justices for all the Welsh counties appears. From this point forward, the laws of England were place as the authority in Wales.
The 12 counties in Wales were placed into 4 sets of three, and a Justices for each group was organized. The work of these Justices were under the supervision of the Court of Great Sessions. A Council of The Marches was held in Ludow and had a superior jurisdiction. This authority extended to Shropshire, Herefordshire, Worcestershire, and Gloucestershire.
The cover to my copy is shown. It was first published 1986 by Routledge & Kegan Paul, London.