Fans pointed at open windows will pull out hot air and allow it to circulate - don't put them IN the window, leave some room by the side for extra air to get drawn in.
If you can't afford cold packs (and if you can, stock up on some NOW), get some ziplock bags and freeze them, with wet washcloths or ripped up towel inside. You want them to be ready in the freezer when you need them. Wrap them in a tea towel and put them on your wrists and ankles.
A big bowl of ice water in front of a fan will blow cold air into the room and make a massive difference. Again, if you don't have ice cube trays get some and freeze them *now*, don't wait until you're already in trouble (although I am melting at quarter past nine but my thermoregulation is bollocks so I don't know if that's the heat or just the me). If you can't afford to get any, clean out empty yoghurt pots or Tupperware or whatever you have that can hold water. Even if you end up with a giant ice cube from a lunchbox, it'll help.
Even just opening two windows at a crack will allow some air circulation - I sometimes prop open my bedroom door and leave the bathroom door open, both windows are locked at a crack because of the cats but it creates a nice little line of wind along the landing.
If you're someone who needs something on them to sleep, take your duvet out of the cover and just use the cover.
Remember that water acts as a lens - you do NOT want to wear a wet t shirt in the burning sun. I did this when I was 8 and if I tan on my back you can still see the scars.
Read the instructions for sunscreen carefully, and use the highest spf you can find. Reapply as per the instructions. If you get burned, Malibu do an amazing aloe-based spray on after sun, I got mine from Savers for 3 quid and it lasts ages. I keep it and any other after sun in the fridge, which means it's incredibly lovely to put on. Aloe is magic for burns so definitely gravitate towards that if you can. My son got badly burned (he's ginger, he went to an outdoor pool and they forgot to give him sunscreen) and he's had 2 helpings of it and you wouldn't believe how much better he is (he couldn't even really wear a shirt).
Make sure your pets have plenty of fresh cold water to drink, and if they usually have dry food consider giving them some wet food for one of their meals (cats are notoriously not always great drinkers but wet food will get them some liquid). Keep the curtains and/or blinds closed in south-facing rooms. I have blackout curtains in the front room and the front bedroom (my son's) and they make a MASSIVE difference to the heat. Make sure pets have access to these cooler spaces if you can create them.
Keep oven/hob use to a minimum if you can. I like making a massive pot of something that can be reheated in the microwave if necessary - the oven especially adds a lot of heat. Or get some wraps and ham and cheese and eat those.
Cordial or fruit juice can be better than water if you're sweating a lot. Cordial is cheap as chips. Salty snacks are also good.
Go to Iceland and buy a billion lolly ices. It's especially a good way to make sure kids stay hydrated.
