There is a detailed entry on “children’s literature” in the Brittanica Library (Ex Encyclopedia Brittanica?). Of UK children’s literature it claims:

The English have often confessed a certain reluctance to say good-bye to childhood. This curious national trait, baffling to their continental neighbours, may lie at the root of their supremacy in children’s literature. Yet it remains a mystery. But, if it cannot be accounted for, it can be summed up.

It also argues that:

In two fields … English post-war children’s literature set new records. These were the historical novel and that cloudy area comprising fantasy, freshly wrought myth, and indeed any fiction not rooted in the here and now.

Of Rosemary Sutcliff’s historical fiction:

There was fair reason to consider Rosemary Sutcliff not only the finest writer of historical fiction for children but quite unconditionally among the best historical novelists using English. A sound scholar and beautiful stylist, she made few concessions to the presumably simple child’s mind and enlarged junior historical fiction with a long series of powerful novels about England’s remote past, especially that dim period stretching from pre-Roman times to the coming of Christianity. Among her best works are The Eagle of the Ninth (1954), The Shield Ring (1956), The Silver Branch (1957), The Lantern Bearers (1959), and especially Warrior Scarlet (1958).