It is generally agreed that the Galfridian 'Camblam' is the Camel River in Cornwall. I myself will not dispute this claim.
There is an old tradition that an inscribed stone found on or near the Camel at Slaughterbridge/Worthyvale was 'King Arthur's Stone:'
This identification of the stone was due to a false reading of the inscription. However, the legend that this stone was Arthur's may, indeed, be very old.
What most Arthurian scholars and amateur enthusiasts alike appear to have missed is that the original name of Worthyvale - preserved as early as the Domesday Book of 1086 - was Guerdevalan/Gerdavalan (Cornish auallen, 'apple-tree', plus garth, "enclosure" - or is guerd/gerd here similar to Welsh garth, "mountain ridge, promontory, hill, wooded slope, woodland, brushwood, thicket, uncultivated land"?). Thus we might surmise that Geoffrey had simply picked up this place-name as the location of the supposed King Arthur's Stone. If so, the happy coincidence that the site bore an apple place-name would immediately have conjured up all kinds of otherworldly connotations.
So, in Geoffrey's mind, Arthur was merely ferried across the Camel from the battlefield to Worthyvale. There is no need to look for Avalon elsewhere.
Worthyvale/Gerdavalan on the Map
The Portal to Avalon (Worthyvale Manor)