How Sea Moderates the British Climate
The reason is that the UK is surrounded by sea, which acts as a massive heat store to moderate extremes of land temperature. In the winter the temperature of the surrounding sea is reliably about 6 - 10 degrees Celsius, enabling a flow of heat to the land if land temperatures should falls below this. Much of Continental Europe is far from the sea, and such flows are much reduced.
In the summer the sea keeps warming well after the longest day of the year - reaching 15 - 20 degrees Celsius. The sea is absorbing heat that might otherwise heat the land, and is helping to keep summer cooler.