Elmet
Elmet is a region better known by conjecture than by fact; as such, it exists in some ways as much as concept as history.
After the Romans left Britain, a number of small British principalities emerged in the 5th century, their number steadily dwindling under the Anglo-Saxon advance. One of these was the kingdom of Elmet, which by the end of the 6th century seems to have occupied much of the West Riding of Yorkshire, from the high hills of the Pennines down to the fertile lands further east. Place-names east of Leeds - Sherburn-in-Elmet and Barwick-in-Elmet - recall its presence today.
Literary sources suggest that Elmet was a strong part of northern resistance to Anglo-Saxon expansion through the 6th century; a ruler named Guallauc was praised by Taliesin the bard for his role. That resistance faltered, however, at the disastrous battle of Catterick in c.600 CE, and Elmet was cut off from its northern allies. Soon after, in around 617 CE, the kingdom of Northumberland marched in and annexed Elmet.
As a British kingdom, that was the end of Elmet, but like its neighbour, Craven, it seems to have survived as an administrative unit under Anglo-Saxon jurisdiction, and full English settlement of the less amenable uplands would have only proceeded slowly. Pockets of British settlements therefore remained in places like the Upper Calder Valley, and are identifiable today by the element 'wal-' (Briton) in place-names, e.g. Walshaw and Walsden; these cases suggest an area rather than a settlement, implying the surrounding area remained mainly British.
'Celtic' is a cultural, not an ethnic, descriptor, encompassing an distinctive set of magical religious practices, otherworldly narratives and vibrant naturalistic art that drew on earlier pre-Roman traditions in Europe. Today, the Celtic idea has been processed through a filter of romanticism to imply an especial mystical aura - but this is where concept, rather than history resides.
Elmet, one of England's last British kingdoms, was history; but as concept it inspired Ted Hughes' vision of the upper Calder valley and its inheritance.