The heatwave is showing no signs of cooling off just yet so you need to be thinking about how your chickens are feeling.
They will be hot - very hot.
They cannot sweat so have to lose heat through their combs or by panting.
You MUST make sure they have plenty of fresh water daily. They will not be eating much but they will drink for England. You will probably notice that egg laying is somewhat reduced. This will be a combination of the heat stress and also because their nutrition is less than it should be because of reduced food intake.
Give them a boost by making up a "mash" of stuff they will always eat, like thawed spinach, frozen sweetcorn, other leafy green veggies or carrots chopped up , a bit of poultry spice for additional minerals. Blend it with a splash of cod-liver oil and moistened layers pellets (or whatever pellets they are on). If you keep the sweetcorn and spinach slightly frozen they will probably enjoy the cooling effect.
Mixed Corn will heat them up so avoid if possible.
Make sure they have enough shade to escape the direct sun. Orpingtons particularly and other very feathery breeds can suffer terribly in the heat. Some chickens also like to paddle in cool water so a shallow tray filled to ankle depth (on a chicken that is) will be welcomed. A greenhouse gravel tray is ideal. You could also cool their water down by putting some ice cubes in with it. If you have poultry electrolytes to put in the water that will help them cope better or a poultry tonic.
Keep an eye on their drinkers so that they are not going green and building up a slimy biofilm on. This is not healthy for them. This will build up rapidly in hot sunny weather.
At night make sure they have enough air. A makeshift temporary door made of chicken wire, very securely fastened to the coop will allow the cooler night air in and keep them safe.