Sorry to hear about the unfortunate, terrible luck of being stuck in traffic and missing a Maiden concert because of it. You did everything you could too, even leaving really early to get there. I can't imagine how frustrating of an experience that was. That's insane that you were stuck on the same 10-20 miles of road for 4+ hours.
Same thing almost happened to me going to my first Maiden concert back in 2008, in Chicago, though not nearly as bad as yours. Had about a six hour drive from where I live, and left really early that morning. Ran into road construction (go figure) around the half way point, and for 30-40 miles traveled bumper to bumper at maybe 10-15 miles per hour through the stretch. Finally arrived in Rosemont and then got lost trying to find the Allstate Arena. Spent probably 45 minutes to an hour just driving around looking for ANY of the markers my map/directions had. This was before I had a smartphone or a GPS so I was basically in the blind. Finally reached the arena, well after Lauren Harris had started the opening act. By the time I finally got in and to my seat, Doctor Doctor started maybe just five minutes later. It was a close call, but for awhile there I thought I wasn't going to make it.
Hopefully they return to England next year and you can see them then. If not, I'm terribly sorry you missed them because of such a shit circumstance.