Just reviving this thread with lots of practical advice and feedback. Please keep it going in the coming days.
Please keep in mind that the cooler you can keep your accommodation in the first place, the less extra measures will your piggies need - and the more comfortable you'll be, too.
Air your house/flat as much as possible overnight and in the early morning and keep the sun off your window glass as much as possible on the sunny side. Close all your windows as soon as outside temperatures get hotter than your indoors. It will get uncomfortable, but temperatures can only climb so much over the course of the day in what is essentially a closed box.
Keep in mind that the air outside is much hotter than temperatures in the shade (up to 40-50 C) and that a strong hot breeze can penetrate into the shade or into a room with open windows with fatal results.
For those in hose pipe ban areas: You can still quickly wet a sheet in the cold shower or use an empty spray bottle filled with water to wet/dampen your sheets as often as possible in front or on your window glass on the inside if you have curtains and don't mind messy windows to create that insulating layer of evaporating cooler air, especially when the sun is on them. You can also dampen curtains on the sunny side but please make sure that they can cope with being wet first. If you cannot create a barrier by the window, you can still peg a damp sheet or towel between on the side of the cage and train a fan on it; please do not train the fan or air con directly on the cage from close by - your piggies could develop URI.
Please bring any outdoors piggies inside again. This time it is a more prolonged heat wave period with temperatures over 30 C forecast for several consecutive days in many parts of England and Wales. Keep in mind that the heat will build up more with every passing day and that your accommodation won't cool down as much as the heat is increasingly absorbed by the walls (and tarmac) as well and reflected at night - that is why city nights are so much hotter than those in the country.
Your piggies will accustom to the higher temperatures to a degree over the course of the week but the frail/elderly, the very young and the pregnant/nursing mothers are at increased risk as their immune system is not working fully and the extra strain on it from the heat can tip a fragile health balance.
Be aware that fly strike can also affect indoors guinea pigs that are not able to clean themselves (arthritis, mobility issues etc). Check your piggies at risk both in the mornings and evenings and see a vet at any time of the day as a life or death emergency.
For our full in-depth hot weather measures and travel advice with guinea pigs (including vet trips), how to spot and deal with heat stroke/exhaustion and fly strike, please see our comprehensive guide and ideally bookmark it:
Hot Weather Management, Heat Strokes and Fly Strike
PS: I am still recovering from my bad bout of Covid and will definitely not be moderating on the forum this month and potentially into the next month until I am full well again.
This also means that I won't be around to host this thread here but you will find lots of your bigger and smaller questions already answered when you read through it and lots of tips tested as to whether they really make a difference.
Please read the guide link above (which is very practical and detailed) and this thread first if you have any questions.
Forum members with their own hot weather care experiences will be able to share tips and help you. Never assume that you know it all already and that what you are already doing could not be improved. One measure may not necessarily be enough.
If your piggies are plastered to any cooling elements, then this means that they are actually feeling overheated. You are actually aiming at your piggies staying away from any colling elements, which means that they are not feeling too hot. Please make sure that your piggies have access to cool, drinkable water at all times.