It depends on how direct you want the help given to pigs to be: the SSPCA (I'm in Scotland so the Sspca applies to me) will go in and help to pull animals in a way that can lead to prosecution.
However, there are numerous guinea pig rescues up and down the UK which all contribute - if indirectly - to helping guinea pigs. Many of them will "pull" guinea pigs from a negligent situation usually because the owner asks the rescue to - in this situation the rescue (Thistle Cavies, Becklen, sophiew's rescue (I'm sorry, I'm not sure of the name of your rescue)) will go in and provide an ultimatium to some degree to not only rescue the pigs CURRENTLY involved but also to prevent the FUTURE of the problem happening again -- normally by saying "Okay, we'll take the pigs, find them a home, heal them - but we're taking ALL of the pigs" (or all sows/boars)
Normally guinea pig-specific rescues rarely end in the prosecution of the owner who neglected the animals, despite that many are large scale. There are some problems and short comings with the law and prosecuting people based on animal cruelty - it's frustrating - and the degree of cruelty that's normally required is disgusitng.
Thistle Cavies is an awesome rescue (and sanctuary) between Dalmally and Oban, they don't advertise or feature on this board unless I or another member feature or advertise them. You can either donate to them, sponsor a pig or buy supplies and trinkets from their shop to support them.
I'm sponsoring Hickory just now, but each month I change the pig I'm sponsoring.
thistlecavies.tripod.com/
Becklen, Sokel rescues range across the UK through their extensive use of fosters. I know they've been involved in a few large-scale pulls, but I'm not sure if they give ultimatiums like TC does.
I can't comment on other rescues, except to say they're there here and aren't hard to find.
Like I said, it depends on how close to prosecution you want to go as to which rescue/shelter you should hit your donation with.
There are also other charitable organisations out there which help animals.