Question: Meirchion Gul was a King of which Brittonic-speaking region of Northern England and Southern Scotland?

Context paragraphs:
Thirteen Treasures of the Island of Britain: The Thirteen Treasures of the Island of Britain (Welsh: "Tri Thlws ar Ddeg Ynys Prydain") are a series of items in late medieval Welsh tradition.  Lists of the items appear in texts dating to the 15th and 16th centuries.  Most of the items are placed in the "Hen Ogledd" or "Old North", the Brittonic-speaking parts of what is now southern Scotland and Northern England; some early manuscripts refer to the whole list specifically as treasures "that were in the North".  The number of treasures is always given as thirteen, but some later versions list different items, replacing or combining entries to maintain the number.  Later versions also supplement the plain list with explanatory comments about each treasure.

King of the Britons: The title King of the Britons (Latin "Rex Britannorum") was used (often retrospectively) to refer to the most powerful ruler among the Celtic Britons, both before and after the period of Roman Britain up until the Norman conquest of England.  The Britons were the Brittonic-speaking peoples of what is now England, Wales, and southern Scotland, whose ethnic identity is today maintained by the Welsh, Cornish and Bretons.

Kingdom of Strathclyde: Strathclyde (lit. " Strath of the River Clyde"), originally Cumbric: Ystrad Clud or Alclud (and Strath-Clota in Anglo-Saxon), was one of the early medieval kingdoms of the Britons in "Hen Ogledd" ("the Old North"), the Brythonic-speaking parts of what is now southern Scotland and northern England.  The kingdom developed during the post-Roman period.  It is also known as Alt Clut, a Brittonic term for Dumbarton Castle, the medieval capital of the region.  It may have had its origins with the Brythonic Damnonii people of Ptolemy's "Geography".

Meirchion Gul: Meirchion Gul was probably a late 5th-century king of Rheged, a Brythonic realm in the area of sub-Roman known as the "Hen Ogledd" (in the North of England and Southern Scotland today).

Coel Hen: Coel (Old Welsh: "Coil") or Coel Hen ("Coel the Old") is a figure prominent in Welsh literature and legend since the Middle Ages.  Early Welsh tradition knew of a "Coel Hen" (Coel the Old), a "circa" Fourth Century CE leader in Roman or Sub-Roman Britain and the progenitor of several kingly lines in the Hen Ogledd ("the Old North"), the Brittonic-speaking part of northern England and southern Scotland.  Later medieval legend told of a Coel, apparently derived from Coel Hen, who was the father of Saint Helena and the grandfather of Roman Emperor Constantine the Great.  Other similarly named characters may be confused or conflated with the Welsh Coel.  The legendary "King Coel" is sometimes supposed to be the historical basis for the popular nursery rhyme "Old King Cole", but this is unlikely.

Rheged: Rheged (] ) was one of the kingdoms of the "Hen Ogledd" ("Old North"), the Brittonic-speaking region of what is now Northern England and southern Scotland, during the post-Roman era and Early Middle Ages.  It is recorded in several poetic and bardic sources, although its borders are not described in any of them.  Some modern scholars have suggested that it included what is now Cumbria in North West England and possibly extended into Lancashire and Scotland.  In some sources, Rheged is intimately associated with the king Urien Rheged and his family.  Its inhabitants spoke Cumbric, a Brittonic dialect closely related to Old Welsh.

Borders Railway: The Borders Railway connects the city of Edinburgh with Galashiels and Tweedbank in the Scottish Borders.  The railway follows most of the alignment of the northern part of the Waverley Route, a former double-track line in southern Scotland and northern England that ran between Edinburgh and Carlisle.  That line was controversially closed in 1969, as part of the Beeching cuts, leaving the Borders region without any access to the National Rail network.  Following the closure, a campaign to revive the Waverley Route emerged.  Discussion on reopening the northern part of the line came to a head during the early 2000s.  Following deliberations in the Scottish Parliament, the Waverley Railway (Scotland) Act 2006 received Royal Assent in June 2006.  The project was renamed the "Borders Railway" in August 2008, and building works began in November 2012.  Passenger service on the line began on 6 September 2015, whilst an official opening by Queen Elizabeth II took place on 9 September.

Bonedd Gwŷr y Gogledd: Bonedd Gwŷr y Gogledd (English: The Descent of the Men of the North ) is a brief Middle Welsh tract which claims to give the pedigrees of twenty 6th century rulers of the Hen Ogledd, the Brittonic-speaking parts of southern Scotland and northern England.  It is attested in a number of manuscripts, the earliest being NLW, Peniarth MS 45, which has been dated to the late 13th century.  The text may have been composed in the 12th century.  The historicity of much of the information is spurious or in doubt.  Although certain parts are in agreement with the earlier Harleian genealogies, the text represents a substantial revision seeking to integrate the branches of many rulers and heroes who are prominent in other traditions, such as the Rheged prince Llywarch Hen.

Acklington Dyke: The Acklington Dyke is an igneous intrusion which extends from northwest of Hawick in southern Scotland east-southeastwards through the Borders region towards the North Sea coast of Northumberland in northern England. The dyke is associated with volcanism which took place at the Isle of Mull igneous centre in western Scotland during the early Palaeogene Period at a time of regional crustal tension associated with the opening of the north Atlantic Ocean and which resulted in the intrusion of innumerable dykes.  The similar Cleveland Dyke has been dated to 55.8+/- 0.9 Ma.  The dyke is composed of tholeiitic microgabbro and basaltThough generally thinner, it is up to 30 m wide in places.  It is one of the most significant of a swarm of such intrusions associated with the Mull centre which extend southeastwards through this region, the others being the Cleveland Dyke and the Blyth and Sunderland subswarms of Northumberland and Tyne and Wear.  They are grouped as part of the Mull Dyke Swarm which in turn is a part of the North Britain Palaeogene Dyke Suite

Urien: Urien , often referred to as Urien Rheged or Uriens, was a late 6th-century king of Rheged, an early British kingdom of the "Hen Ogledd" (northern England and southern Scotland).  His power and his victories, including the battles of Gwen Ystrad and Alt Clut Ford, are celebrated in the praise poems to him by Taliesin, preserved in the "Book of Taliesin".  He became the "King Urien of Gorre" of later Arthurian legend and his son Owain mab Urien was later known as Ywain.

Supporting paragraphs: Meirchion Gul Rheged
Answer: Rheged