In the last decade of the eighth century, Norse raiders sacked a series of Christian monasteries located in what is now the United Kingdom, beginning in 793 with a raid on the coastal monastery of Lindisfarne on the north-east coast of England.
From 865, the Norse attitude towards the British Isles changed, as they began to see it as a place for potential colonisation rather than simply a place to raid. As a result of this, larger armies began arriving on Britain's shores, with the intention of conquering land and constructing settlements there.
https://en.wikipedia.org/wiki/Viking_activity_in_the_British_Isles
Great Heathen Army (865–78)