BREAKING: the September 10 La Palma event WAS a Trident missile test!
Based on an analysis of his photographs, I concluded that what he serendipitously captured was most likely an unannounced SLBM (Submarine Launched Ballistic Missile) test in the mid-Atlantic, possibly a US or British Trident test.
News has just broken that the US Navy in cooperation with Lockheed-Martin indeed conducted a test with a modified Trident II D5 Submarine Launched Ballistic Missile in the mid-Atlantic on September 10 (and again on September 12). The Trident D5 missile was launched from a submerged Ohio-class submarine.
(Tip of the Hat to Brian Webb who first brought up the confirmation news today at the SeeSat-L list)