I'm in the UK and I beg to differ from the views above. Maybe it's just because I'm from Scotland, but keeping guinea pigs outside IS a very dated view on things. Cages in pet shops are, of course, sold as indoor cages. I very rarely see a "guinea pig outdoor hutch" for sale in pet shops now. Everyone I know either keeps their guinea pigs indoors or has some sort of insulated, temperature controlled outbuilding for them. The books I've had since childhood stated that the optimum temperatures in which to keep
cavies was the same as that of "human" room temperature. Most rescues up here will not rehome to outdoor homes.
I don't believe the temperature here in Scotland is much more extreme than that of England. It's just a case of you hadn't heard of keeping guinea pigs indoors - it's not that it's "not the done thing" over here.