I take it (both from references in the book and poking around on Amazon) that the authors have written quite a bit about ancient British mythic themes, of the Land, Kingship, and the Under/Otherworld, and these weave through Legendary Britain, freely mixing with "historical" material and analysis of surviving legends and other cultural traces, giving it almost a "dream-like" take-away, as history and myth and legend and places and names all blur in this particular telling.
As one would guess by its sub-title "An Illustrated Journey", this is a progression through various British sites, from Cornwall in the south up to the Orkneys in the far north, making 10 stops along the way, chosen more for their thematic elements of Prophecy, Kingship, and the Land, than for their fame as archaeological zones. For instance, Stonehenge is skipped over, and Avebury only mentioned in relation to other sites, yet Bath has its own chapter.
Characters from the legends, Arthur (along with relatives and associates), Robin Hood (with others), Merlin in his various forms, Tristan & Isolde, and various denizens of the Under/Otherworld, most notably the Fairy Queen, are the main element here, weaving in and out of the "historical" and evolving as the re-telling of centuries passed. Each chapter ends with a "legend" re-imaged by the authors, be it of The Smith King of Wayland's Smithy or of an ancient hermit of Iona, which brings together bits and pieces from mythic remnants, historical elements, and archaeological contexts.
Of course, this stuff is rather "hereditary" to me, so I get a serious hankering to visit these sites when I read this sort of material (I had a couple of books as a wee lad which dealt with visits to the Faerie Realm which touched me deeply), but "your mileage may vary". I'm not sure how coherent this book would be for one who had not had at least a general familiarity with the long-view history of the British Isles, and the archaeological remains and legends of the culture. If one only knows the "Disney" versions of Robin Hood or King Arthur, one might be rather confused at the elements introduced here!
Legendary Britain is, however, a rather unique book and is delightful within the strange context it creates for itself, while "neither this, neither that" as far as the component parts, it does pull together a particular telling, which it stays true to all the way through. Currently out of print, this hardcover is available via the Amazon new/used vendors fairly reasonably with a "good" copy going for just over $4 and a "new" copy going for just under $8 (both of those plus shipping, of course), so if this sounds like a literary journey that you'd like to make, it's available!