I'm a casual collector. I don't necessarily buy games just becuase they're rare, I tend to just buy games that I like. But with Gameboy games I'll usually try and go for boxed complete as the boxes and manuals were often thrown away (more so than with some other cartridge games because the paks were made to be portable and take up less space) but I'm struggling to know where to look other than eBay.

Craigslist is as good as non existent and things like gumtree rarely show anything of interest. I know a few retail stores but they're not the cheapest and I've struggled to find charity shops or car boot sales that have anything other than PS2/Xbox360/Wii. 

Can anyone share their Alladin's caves where I might be able to pick up some Gameboy games for reasonable prices?

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.

They do exist but the prices aren't that great. 

Forums may be the way to go.