The main reason for this is the sea. Because the oceans are so much better than the land at retaining heat, the sea temperature varies far less from season to season. Off the Cornish coast, for example, the average temperature varies only between 10C in winter and 16C in summer. A good deal of this effect is due to the prevailing south-westerly winds created by the effect of the earth's rotation on the exchange of warm and cold air flowing from pole to equator. Those south-westerlies bring a continuous supply of warm water to these shores from the south Atlantic. This is what used to be called the Gulf Stream, but is now known as the North Atlantic Drift. It also explains why our coldest waters in winter are not, as one might expect, off the coast of Scotland (which is warmed from the same source), but farther south, around Norfolk.
In general the seas around Britain serve as liquid lagging, insulating us from the worst of Continental weather conditions. Even when cold winds come from the east or north, they are bound to pick up some warmth and moisture as they blow over the waters.
The winds competing for air space above Britain come from three main directions: the Arctic, continental Europe and the Atlantic, of which the last has a natural advantage since it is a constant effect of the earth's rotation. Atlantic winds, having blown over all that ocean, will be much the same temperature as the water itself, warming our winters and cooling our summers.
Easterly winds, after their long journey over land, can be bitterly cold. I remember when I was at Cambridge being told that the icy winds came direct from the Urals, with no hills intervening to obstruct their progress. That sounded convincing, until I looked on a map and discovered the Central Russian Uplands. However cold and flat it may or may not be between Cambridge and Chelyabinsk, the main reason why Kent and East Anglia are so cold when the east wind blows is the narrowness of the waters between there and continental Europe. Even the easterlies that cross the cold North Sea are warmed more on that journey than those destined for Cambridge.
While the North Sea crossing from Yorkshire to Denmark is about 600 km long, the waters between Scotland and Iceland stretch for 800 km. They therefore have a good opportunity to warm the north winds from the Arctic before they get to us - which is why the northerlies, while always chilly, are not nearly as bad as they might be.
With weather, however, nothing is simple. That nice North Sea crossing that warms the air for Yorkshire also results in more cloud, making conditions feel colder. And you cannot even trust a wind to come from where it pretends to. Our northerly winds generally do not blow direct from the Arctic, but start by blowing east across the north Atlantic, then turn south. And southerly winds may be anything from hot, dry air direct from the Sahara to very cold easterlies turning right to chill the south of England.
But wherever they come from, the winds have to cross the sea to get here, and while all that moisture in the oceans may bring us a continual threat of scattered showers, the main effect of being an island is to take the chill off our worst weather.