During part of the Old Stone Age (Palaeolithic) there was an Ice Age. This meant that 500,000 (half a million) years ago Britain was covered by a massive sheet of ice! As this Ice Age ended the temperature increased and the ice melted, as part of natural climate changes.
By the end of the Old Stone Age (Palaeolithic) all the land under the ice was available once again for people to live in. The there was no Britain though – as we were still joined to the rest of Europe! You could literally walk from what is now Britain to France, Belgium, the Netherlands, Germany, Denmark or Norway!
Here the sea is shown in blue, the ice in white and the land in green.