During the last ice age*, ~70,000 to 20,000 years ago, the climate was much colder in Europe.
As a result, the northern part of Europe was fully covered by the Fennoscandian (a.k.a the Scandinavian ) ice sheet, which extended up to the British Isles and some parts of Poland and Germany. In central Europe, the Alps were also almost fully glaciated.
The storage of all this ice on the continent lowered the sea level (seedark green), which substantially reduced the extent of the North Sea.
*This period is referred to as the Weichselian glaciation and the Würm glaciation in Northern Europe and the Alps, respectively.
A more complete and accurate dataset (including GIS maps) of Europe during the last glacial maximum is freely available :
Becker, D., Verheul, J., Zickel, M., Willmes, C. (2015): LGM paleoenvironment of Europe – Map. CRC806-Database, DOI: http://dx.doi.org/10.5880/SFB806.15