Defender of the Realm, by Mark Huckerby and Nick Ostler (Scholastic, March 28 2017) might be just the thing!
Alfie is a pretty ordinary boy, overshadowed by the charisma and accomplishments of his younger twin brother, Richard, the golden boy of their form at the English boarding school they attend. But for all of Richard's gifts, Alfie overshadows him in one very important way--as the older twin, he's the one who's the heir to the throne of Great Britain. And when his father dies in a most utterly unexpected way, Alfie inherits everything. Including the unfinished monster slaying business that went so wrong for his father.
For the kings of Britain are magical defenders of the realm, with a hereditary magical flying horse and suite of weapons. Alfie is forced to scramble not just to get his head around being king, but to learn to fight for his country as the current Defender (neither of which appeals. He has trouble believing he could do a good job of either. But the monstrous lizard dragon thing that killed his father is still out there....and Alfie must slay it.
An ordinary girl, Hayley, finds her own ordinary life disrupted when she sees the monster for herself, and witnesses Alfie's father's early attempt to defeat it, taking home with her one of the creatures scales. She believes in the Defender of the Realm, and is happy to help Alfie as best she can....And he needs a friend he can trust, because sadly there are those working to bring him down. She makes an excellent side-kick for him, with her confidence bolstering his faltering efforts to become the true hero he needs to be.
Many and various excitements ensue, as Alfie races to secure the magical wards of Britain before his adversary seizes them and become invincible. It's not tremendously Deep, and doesn't dramatically break any new ground, but it is just fine for what it is--a magical, mythical adventure story whose pages turn quickly and pleasingly in fast-paced jaunts around Britain, with some thoughtful elements of character growth.
(Hayley is mixed-race, of Jamaican heritage, adding diversity. Though primarily a side-kick, she's character enough in her own right and as a point-of-view protagonist to count this in my Diverse MG and YA speculative fiction list.)
disclaimer: review copy received from the publisher.