Well, you can get a GBASP, the latest models, for about £60 if you order an import. Then all you need to be able to play it here in the UK is an UK adaptor which shouldn't cost more than £5. And don't worry, every single Gameboy ever made is universal and therefore, PAL (Eg. UK) and NTSC (eg. Japan and USA) games will all work on ANY Gameboy. A GBASP will also play Gameboy Color and the original Gameboy games since it's backwards compatible, much like the way your PS2 can play PS1 games.
Games can cost a bit, especially latest releases which cost a maximum £35 but since the GBASP has so many great games, you can get some classics like Golden Sun 1 and Advance Wars 1 for less than £20.
Oh yea, don't be bullied by others to not bother with the GBA SP and be pursuaded to wait for the 'oh-so-great' PSP, Sony's planned handheld. The PSP might be a gazillion times more powerful but what makes a console great is it's games, not it's specifications.
I have a GBASP and I absolutely love it. I'm addicted to Final Fantasy Tactics Advance and Fire Emblem at the moment. The GBASP has most of the good RPGs out there at the moment which is one major reason why I got it.