I don't know much about gameboy collecting but for alternatives I would suggest trying facebook groups or just online forums like this. Also, if you network properly you might become the go to user who buys boxed gameboy games and you might get random messages and first dibbs.
maybe try to go to retro game fairs (if they exist in the UK) i know some do in mainland europe.
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Currently Playing: The Legend of Zelda: Breath of the Wild
Recently Completed: Shovel Knight