The City's Son by Tom Pollock

While Beth is running from her ordinary life, she stumbles across Fil's path and is introduced to the world of magic living within London's streets. Although her new life on the streets is far more exciting than high school, Beth quickly learns of the dangerous monster Reach who is trying to destroy everything that her and Fil love so much about London and replace that with his cranes and modern buildings, and the pair unite to start a war.