Why not also try the Pet Shopping page for information on where to buy stuff for your pet.
Cavy Cages has loads of advice on making or buying a great home for your piggies.
Appeal: Could you offer a rescue guinea pig a new home? Please visit Guinea Pig Rehome to see guinea pigs desperately needing homes in your area. Many people just loose interest in their pets and dump them on rehoming centres which are full of beautiful piggies needing home, please rescue rather than buy from a petshop.
Treen's Pigs is mainly fun stuff like pics, animations and games,but there's an information section too with lots of useful stuff. This site also has a great page on building a cage like the ones on the Cavy Cages site with instructions and photos. Has an amazing collection of links that are well classified so that you can find any information that isn't available on the site and lists a very large number of UK rescues.
If you are thinking of breeding guinea pigs then read this page from Cavy Spirit first.
Barmy4Boars is a website dedicated to boars. It has lots of really useful information about keeping boars together and other issues specific to male guinea pigs. Worth a look even if you have female guinea pigs though!
Aviary Accessories do excellent rabbit hutches although the indoor guinea pig pens on the Guinea Pig Cages page are a better idea, especially in the winter. We put our guinea pigs out for the summer and now have one which can be seen on the Guinea Pig Cages page.
Cavy Madness. Not a UK site but this a great website with lots of information and a cavy community including a message board and pig-pals a system to pair up inexperienced owners with a pen pal for advice. I love the idea. The discussion board is my favorite guinea pig board as it is very friendly, and has many UK based members. It also has a rescues and adoptions section that often has piggies in the UK.
Galen's Garden - do an interesting range of herbs and natural pet health products.
Gorgeous Guineas - essential oil soaks and shampoos for guinea pigs with skin problems.
The Winking Cavy - a good shopping site with lots of cavy and other small rodent themed gifts. I particularly like their guinea pig rubber stamps and rodent jewellery.
Squeaky Pigs Rescue based in West Yorkshire always has piggies needing a home. This is also great site with an online shop for piggie gifts.
Pet Rescue Forums - links and forums for people interested in adopting an animal or who have animals needing a home.
Guinea Lynx Cavy Health Care Online! A medical and care guide for guinea pigs. Organized by main topics with links to off-site advice. This is the place to go for health information. It also has forums which have very experienced owners on them but be warned, this forum isn't quite as friendly as some. However if you need information on something like an obscure medical condition then don't be put off, they will help.
The Guinea Pig Website - "Information, Care Tips, Links, Pictures and Plenty of Cucumber" - Well worth a visit.
Cavy Rescue has guinea pigs and many other rodents up for adoption.