I did the same thing. Picked up Smash Bros Brawl on the GAME website pre-owned for £7.99, but it came to £4.50 when I used points I had still saved on my reward card. Definitely a great Wii game to own for that price!
As for CEX; I wouldn't count on it happening. I worked there for 5 years, leaving in October in this year to focus on my final year of Uni more. They use a centralized system and never, ever run sales. In fact, they try and up the prices of games as much as possible during this period.
Skyward Sword does seem to be cheapest in Cex though (at least for the moment). It's also a good place to pick up a Wii Classic Controller if you ever plan on getting any Wii Virtual Console titles (I used a lot of my Club Nintendo Stars to get Wii Points and put them all towards buying Majora's Mask). Places like Tesco, Argos and Game sell the white, original classic controller for £15, and the Pro version for £25. Cex sell the white one for £8 and the Pro for £10. Official ones, too.
Nintendo Network ID: MikalM
Games owned: ZombiU, Nintendo Land, NSMBU, Darksiders 2, Black Ops 2, Trine 2.