In the Keep of Time

The linked review by Kirkus begins, "When four English children are transported through time, via the magic key to an old Scottish keep, they react very differently: ..." It specifies both 1460 and "the 22nd-century ruins of technological civilization". The children are 5 to 14 years old. The 15th-century episode makes nothing of their English "nationality which is, after all, the same as that of the villagers' enemies."

1st ed. Publication date 1977-03-01 from the Kirkus Reviews issue with the same date, as reprinted online (see title record). Price from: https://www.newspapers.com/newspage/25586063/.