My speculations are over at Albion Awakening.
The original inhabitants of the island of Albion - or Merlin's Enclosure, as it was first called, Merlin being the presiding deity - were a race of Giants. Indeed, the name of Albion comes from their king.
When the island was first settled by normal-sized Men (Brutus the Trojan and his followers - great grandson of Aeneas) - it was necessary to defeat resident Giant population. Gogmagog was the most famous of these - apparently a small and weak example of the race. However, a remnant of Giants continued as an occasional menace for a very long time afterwards.
Giants crop-up in many historical, religious, legendary and mythical sources, from all over the world; so there is no good reason to doubt their reality - except that we don't seem to have any nowadays. Much the same applies to the races of elves/ fairies and to dwarves - there is ample evidence for their existence in earlier time; far more evidence than for most supposed facts of history.
But of course, that does not mean that Modern Man would be able to perceive Giants, fairies or dwarfs, even if they were present - since we are self-blinded to much of the primary reality of this world; and furthermore treat as dogmatically-real many things which are imperceptible and undetectable (except by long chains of insecure and labile inferences).