By now it is getting cold again. The Eglu is well insulated, and if it is also packed with hay, your guinea pigs will be cosy. There will be less grass around, so it's back to the supermarket again! Guinea pigs should not be put out on frosty grass, as this will cause digestive upsets (similarly, salad should not be fed directly from the fridge).
Remember that the drinking water may freeze, and need to be replaced more often. It is often easier to have two bottles, and rotate them, bringing a fresh one from indoors each time, rather than having to crack the ice each time. It may be worth considering adding some extra vitamin C to their drinking water at this time, either in the form of sugar-free ribena, or as soluble vitamin C tablets.