I wonder if this brythonic version of the name for London was what the Saxons based the modern word from, or visa versa.
It looks more likely to have been one of the Norman variants as this article '''http://londonist.com/2014/01/how-london-got-its-name
suggests:- "Following the Norman conquest, records begin to show the area referred to by its modern name, or similar versions such as Lundin, Londoun, Lunden and Londen. Over the centuries, the spelling settled down on London. "
The Roman Londinium seems to be the earliest certain name - anything prior to that is probably guesswork! And the Modern Welsh Llundain may well come from other, earlier versions of the name - Welsh has changed a lot over its 1500 or so years.