Hi Emma!
Thistle Cavies are the only dedicated Scottish rescue I know of. I adopted two of my pigs from them, and several other members here have adopted their piggies from them too. It isn't close by for me (a three hour journey each way), but worth it!
There is a small rehoming centre near me (SW), but they rarely have guinea pigs. Freecycle occasionally 'comes up' with pigs, your local newspaper, vets office or (pet) shop notice board are also places to keep looking.