The Vikings
The Vikings were at one point the scourge of the country yet their influence has been widespread (and still exists) across the centuries. This invading race was known as the Norse and comprised people from Denmark, Norway and Sweden. Most Vikings, although seafarers probably had some other trade as well (i.e. farming).
It is thought that the first raids by the Vikings in Britain took place in the 8th century and that they initially targeted monasteries that had little protection and that held a lot of wealth. By the end of the 9th century the Vikings had set up various large settlements all over the country and were a significant political influence.
For example, in the second half of the 9th century the Vikings took over York and established settlements (often by seizing existing farm land) all over Britain and in parts of Scotland/the Scottish Isles.
During the 9th and 10th centuries Alfred the Great did make some in-roads into Viking occupation and drove them into the eastern part of Britain where he allowed the Vikings to set up their own society (known as Danelaw) for a period of time before they were driven away from that region as well. Whilst Danelaw existed the British would pay the Vikings money (Danegeld) to keep them from further attacks.
But, the Vikings returned and by the middle of the 11th century both England and parts of Scotland and Ireland had Viking rulers (King Canute and Earl Thorfinn Sigurdsson respectively). The Norman Conquest of 1066 saw the end of Viking rule and a subsequent decline in their influence in Britain. The Vikings were driven out of Scotland in the mid 1100s and out of the Scottish Isles by 1468.