I dont think they have stopped making Gamecube games. After all, apart from Zelda, there isnt anything noteworthy on the Wii (you play GC games on the Wii). With that in mind, it would make sense for GC games to be available, until the next lot of Wii games comes along.
GAME isn't very good for GC games. Try Gamestation, if you can (there's one in Romford, if you're prepared to travel). Or try some independent shops.
The internet is also excellent.
Whereabouts in East London are you from, if you dont mind me asking?