The authors need to locate all the geographical names of the Brut in Wales, so they actively seek candidates, based partly on the similarity of modern names and partly on the hints given in the Middle Welsh sources. They state that there is only one possibility for identifying a Temys in Wales: the River Teme, a tributary of the Severn. Whilst the modern name certainly contains the element *tam- that would produce Tem- in Middle Welsh, the -ys ending of Temys requires some sort of suffix in Brittonic, either *‑īsā or *-ēssā. In fact, the River Thames contains such an element, as it derives from Brittonic *Tamēssā, and there are Old English forms of the name with the spellings Temis and Temes that parallel the Middle Welsh form Temys precisely.
Their search for Llundain takes them back to Geoffrey of Monmouth, where the name Kairlud is translated Londinium in Latin, but they argue that the name is preserved in Ludlow, on the River Teme. However, Ludlow is recorded in the twelfth century as Ludelaw, a name containing Old English –hlæw (‘hill’) and a prefix that appears to be Old English hlūd (‘loud’), probably a reference to the River Teme.
Kairlud is apparently an invention of Geoffrey, as it is not found in earlier writers. Llundain, however, has a clear derivation that, despite the most superficial of similarities, cannot be connected with Ludlow. Llundain occurs as Cair Lundein in Old Welsh (the Harleian MS of the Historia Brittonum Chapter 66a has Lundem in error, which has made its way into the printed editions, while others have Lunden). This Old Welsh form gives Old English Lunden and itself derives from a Brittonic *Lūndonion, attested as the Late Latin form Lundinium in Ammianus Marcellinus’s Res Gestae (xxvii.8, xxviii.3 and xx.1), the earlier Londinium, London.