Dvd.co.uk are selling "Harry Potter and the Order of the Phoenix" for the Nintendo Wii for £15.99 (inc Del.)
You can get cashback through the usual sites (e.g. 4% Quidco, making it just £15.35).
The next best price seems to be £19.99 from Woolworths.
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 is in denial about the teenagers recent encounter with the evil Lord Voldemort, preferring to turn a blind eye to the news that Voldemort has returned. Fearing that Hogwarts venerable Headmaster, Albus Dumbledore, is lying about Voldemorts 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 Umbridges 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 'Dumbledores Army,' Harry teaches them how to defend themselves against the Dark Arts, preparing the courageous young wizards for the extraordinary battle that lies ahead. With the ability to play multiple characters, including Harry Potter, Dumbledore and Sirius Black, the videogame of Harry Potter and the Order of the Phoenix offers fans the opportunity to wield a wand, explore all around Hogwarts, and experience one of the most exciting and dangerous years in the life of the Boy Who Lived.