ShopTo is selling "Harry Potter and the Order of the Phoenix" for the Xbox 360 for £17.99 (inc Del.)
You can get cashback through the usual sites (e.g. 4% Quidco, making it just £17.27).
The next best price seems to be £24.93 from TheHut.
In Harry Potter and the Order of the Phoenix, Harry returns for his fifth year of study at Hogwarts and discovers that much of the wizarding community has been denied the truth about the teenager's recent encounter with the evil Lord Voldemort. Fearing that Hogwarts' venerable Headmaster, Albus Dumbledore, is lying about Voldemort's return in order to undermine his power and take his job, the Minister for Magic, Cornelius Fudge, appoints a new Defence Against the Dark Arts teacher to keep watch over Dumbledore and the Hogwarts students. But Professor Dolores Umbridge's Ministry-approved course of defensive magic leaves the young wizards woefully unprepared to defend themselves against the dark forces threatening them and the entire wizarding community, so at the prompting of his friends Hermione and Ron, Harry takes matters into his own hands. Meeting secretly with a small group of students who name themselves "Dumbledore's Army," Harry teaches them how to defend themselves against the Dark Arts, preparing the courageous young wizards for the extraordinary battle that lies ahead.
Discover the real Hogwarts: Explore the Hogwarts from the movie and discover its magical secrets using the gesture-based spell casting system. Talk to the portraits, complete their missions and gain passwords for shortcuts around Hogwarts. The player can interact with virtually everything they come across, gain discovery points and unlock secrets in the Room of Rewards.
Recruit Dumbledore's Army: Find and recruit the members of Dumbledore's Army. Harry Potter and the Order of the Phoenix's nonlinear gameplay allow the player to choose where they go, what they do and when they do it, within the magical surroundings of Hogwarts.
Attend Hogwarts classes: Complete side-missions for teachers and attend Herbology, Charms, Transfiguration and Potions classes to earn O.W.L. grades. Achieving "Outstanding" opens secret areas of the castle.
Play Wizard Games: Compete in Exploding Snap, Gobstones and Wizard Chess games throughout Hogwarts and challenge the best from each house.
Experience the movie in the game: Play the key moments from the movie, from the Dementor attack in Little Whinging to the ferocious battles at the Ministry of Magic. Experience the most authentic-to-the-movie game experience in Harry Potter videogame history: the castle is built from the same blueprints used to build the movie sets
actual movie assets were referenced to create a more authentic feel
many of the movie's actors have been head-scanned and contribute their actual character voices
and the key Harry Potter music theme can be heard in this videogame.
Play other favourite characters: Fly Fred and George out of Hogwarts as they escape Umbridge's regime. Play as Sirius Black to protect Harry as you confront Bellatrix Lestrange and Lucius Malfoy in the Ministry of Magic. Play as Dumbledore in his momentous final confrontation with Voldemort.