HI, I have always had cavies in the UK and they do fine outdoors. My sister currently has two who are loving life. Not sure if you have already stopped checking here but just in case, off ground cages are really good to help with the damp. My dad usually makes the cage out of wood with an angled roof (sloping backwards). It is made the accepted size ...not sure now though what that is. I think 2x2 ft per each GP????Better check that one!!! He then gets a piece of roofing felt which he nails to the top with overhang. We then usually cut some clear (to let the light in) plastic sheeting with a length of thin wood (for weight) on the bottom and top. We nail the top to the top front of the cage and then in bad rainy weather we pull it down over the front of the cage. We have also during the cold put a piece of carpet between the plastic and cage to insulate overnight. We also pack the cage with hay which the cavies love to eat their way through and has the added benefit of providing a little warmth.
This has always kept our cavies safe and warm and locks and latches on all of the doors have made them fox proof. A big windowless bedroom is a good idea for a safe haven for you pets. Luckily we have high fences around the garden so the GP's are allowed a free run all day which they take full advantage of and now go back in their cage at night on command! It is possible to easily build an enclosed run though out of reinforced chicken wire( the stronger the better to prevent foxes) and a wooden framework with lifting lid. They just need a little hide home in there. These solutions always work out much cheaper than buying and the guinea pigs seem to love it!!My first gp's lived for around 8 years.We have kept some ever since..... hope this helps...great childrens pets!