The plant is named after the local town of Mt. Egerton. The growers had a choice of “Egerton Purple” or “Egerton Blue” and decided that the latter flowed better.1
Angustifolius is Latin for "thin"or “narrow”.2
1 Holmes, Rosemary. ‘Australia: A Lavandula cultivar’ Garden Plant Conservation Association of Australia, Inc.
2 Latin Dictionary