What is the origin of UK place names?

The UK has a diverse and sometimes bewildering array of place names. They originate from Celt, Roman, Anglo Saxon, Viking and Norman settlers.
In 1066, the Norman invaders forced the existing Pagans to the periphery of the country. The Normans made only minor impact on UK place names. Even the Romans were largely impotent at affecting place names. (300 terms of Roman origin being commonly used).
The earliest UK place names relate to rivers where settlements originated. The Celts added descriptions to these. Gaelic speaking tribes occupied much of Wales and Ireland. So Aber meant the mouth of a river e.g. Aberarth in mid-Wales. A cair referred to a fortified town e.g. Carlisle in Cumbria and glen, a narrow mountain valley, e.g. Glencoe (Scotland) through which flows the River Coe.
The Anglo Saxons succeeded the Romans. Consequently, Old English terms are found in places where Saxons settled in the central, south and west of UK. Didcott (Oxfordshire) deriving from cot meaning cottage: Swindon from dun meaning hill and Birmingham from ham meaning homestead.
The Vikings too had their influence on the names of places they plundered. Ormskirk derived from kirkja meaning church and Patterdale (Cumbria) from dalr meaning a dale or valley.