Your links do not say that the white cliffs of Dover are Jurassic. The cliffs consist of chalk, and
The Penguin Dictionary of Geology defines
Chalk as 'Strictly, the very fine-grained pure-white limestone found in the Upper Cretaceous of Western Europe.' The British Regional Geology handbook for The Wealden District describes the Chalk under the Cretaceous System in Chapter 4. 'Strata Exposed at the Surface - Mesozoic Systems'; on page 43 it says, 'Good sections in the Middle Chalk are to be seen in the Dover cliffs'.
According to
Cretaceous - Wikipedia , 'In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the White Cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast.'