Al says that there was no intention of flying through the bridge spans until he saw the bridge appear in front of him. His intention was to overfly Parliament and London because of the way in which the RAF brass were heroically ignoring their own anniversary. He did this, but on the outbound route he saw the bridge and suddenly realised that he could fly through it. His only excuse for such wild thinking was that he had a cold at the time and was suitably medicated!
It's fine to look at all the risks involved but my view is that he showed some flair and enthusiasm and wasn't afraid to take a calculated risk. These days, in a country where it'll probably be declared illegal to get out of bed without a Health & Safety assessment soon, we need more people like him.