This is a topic which at one time was of great interest. In recent times that interest seems to have abated.

There are some interesting immediate possibilities of the origin of London. The Celtic word dun, meaning fortress, appears in a number of place names in Britain and Ireland. There is also the Celtic word lin, meaning pool. The name Dublin derives from the Celtic for black (dark) pool. The name of the city Lincoln was originally called by the Celtic name Lindon. The Romans called it Lindom when they established a fortress there. Later the name was changed to Lincoln when the fortress-town became a colonia, a settlement for retired (time-expired) Roman soldiers. The name Liverpool came from the Norse name meaning the pool at the slopes, where slopes referred to the banks of the River Mersey.

However the one thing that is known for certain about the origin of the name London is that it came from the Roman name of the town Londinium. Therefore any explanation of London must account for the suffix ium.

The one most authoritative explanation of the origin of the name London was developed by Richard Coates of the University of Sussex. Coates argues that London came from the pre-Celtic name for the lower part of the Thames River, Plownonida. This is Coates suggestion of what the river was called in the pre-Celtic language. Coates based this on Indo-European root words that appeared elsewhere in Europe. Such a name would have meant, according to Coates, boat river or swim river. More simply plownida as a place name probably would have meant a place where the river widens and could only be crossed by boat or swimming and not by wading. When the Celts occupied the area they accepted the place names but they could not pronounce the initial p in Plownonida so the river became Lownida and the settlement Lownedonjon. Coates' explanation has certain virtues, but since it is founded entirely upon a hypothetical pre-Celtic name for which there is no other evidence, it is not universally accepted.

All of the other explanations of the origin of London seem to be shear fantasy without any historical basis. One was that a King Lud captured the city and insisted it be named after him. Another was that a descendant of Aeneas came to Britain and established New Troy.

Sources:

Richard Coates, "A New Explanation of the Name London," Transactions of the Philological Society, vol. 96, no. 2 (1998), pp. 203-229.