As I have previously noted; the original inhabitants of Britain were said to be giants; and there is considerable 'documentary' evidence for the reality of giants.
But if we accept that there once were giants, then of what nature were they? Presumably not gigantic Men - since that seems biologically impossible - but if not that, what were they?
Bearing in mind that the age of giants was also the age in which magic was a part of everyday life; one interesting idea comes from John Michell in his beautifully-written superbly imaginative New light on the ancient mystery of Glastonbury (1990) - edited excerpts of pages 51-2 follow:
The giants of old undoubtedly symbolized natural forces, but we are repeatedly informed by early historians that sometimes they were visible in monstrous and grotesque forms. They belonged, therefore, to the cryptic category of beings which includes the yetis, Sasquatch and Bigfoot.
These creatures are occasionally seen and heard, but essentially they are phantoms. They represent an aspect of the 'genius loci'; the indwelling spirit of certain mountains, lakes and wild places. Their forms are fluid and adaptable, being determined by the collective imagination of local people.
In traditional societies, magicians and shamans know about such things, and are sometimes able to invoke them.
It is often easier to produce phantoms than to dissolve them. Like all 'technologies' they had unwanted side effects. In the course of time, they become more solid and may even bleed when wounded.
Therefore, not all the spirits raised by magicians were properly laid to rest, and some lingered about the countryside to establish a breed of monsters. This may be the origin of the giants which were apparently still to be found in Britain in the second millenium BC.