Found these images from Avengers Arena #8 on 'CBR' which shows a decapitated Kid Briton! So now we know how he died, and I must admit this angered me somewhat, and I'm on the verge of cancelling my order for the Avengers Arena Graphic Novel. The other characters in Braddock Academy just do not appeal.
The build up to his death goes something like this...
Taken from the origin details on Comic Vine connected to another Braddock Academy character called 'Anachronism'.
Aiden used to be a chubby young man, interested in video games and eating fast food. He then got the body of an immortal Celtic warlord. His chosen codename, Anachronism, means "someone or something that belongs to an earlier time". Before Murderworld and his new body, he accidently walked in on Kid Briton cheating on his girlfiriend, Apex, with fellow Braddock Academy student Nara. Kid Briton threatened to beat him up if he told anyone. It would appear that this incident caused Aiden to inherit his new body. When he inherited the new body, Nara began to flirt with him and the two came close to kissing before Kid Briton fought with him again. When he and the rest of his friends were captured by Arcade, Aiden and his best friend Bloodstone were caught in the middle of a plot started by Apex to get revenge on Nara. The two of them fell off a bridge into the water and Nara was subsequently blasted into the water by Death Locket, who was swayed into doing so by Apex. Eventually, Arcade set it up so the Braddock Academy would reunite and Nara began to attack Apex. Apex manipulated Deathlocket into attacking Nara and Anachronism but then Bloodstone blasted Deathlocket out of the way, revealing the plot. Apex then swayed Brian into saving her, who then began to attack and dealt a grave wound to Nara. Before he could finish her off though, Anachronism intervened and decapitated his rival.