Fireball

Two cousins are thrust into a parallel reality when they travel through time and fire in the first book of the alternative history Fireball Trilogy, from critically acclaimed Tripods author John Christopher.
Simon and his cousin Brad hadn t wanted to spend the summer together. And the quiet London suburb where Simon lived was the last place they expected anything unusual to happen.
Then the fireball came.
The huge whirling ball of light was like nothing the boys had ever seen before and they felt compelled to investigate.
They awoke in a world where everything had changed. It's still England...but an England terrifyingly different from the one they had left behind. They passed through the fireball, and now they will have to learn to survive in the fascinatingly strange world on the other side "