Kan det här vara "Framtiden tog oss" av David Severn?
Two English schoolboys are suddenly transported from the mid-20th century to the end of the third millennium. The Sussex terrain in which they find themselves is the same, but much changed, apparently post-nuclear apocalypse, with a much reduced population living as the Anglo Saxons might have. The boys journey to London, where they quickly become involved in a popular uprising against the ruling elite, who cruelly suppress the proletariat, whilst themselves enjoying a technologically advanced and highly mathematical existence. It was the Calculators who arranged for the boys to be whisked from their headmaster's study at a critical moment - although to a degree that was an accident; their intended target was the headmaster.