Air circulation is no use if you're just circulating hotter air - you may as well have a heater on. Close internal doors as well as you may find some rooms stay cooler than others.
Thinking of it like a flask keeping contents warm or cool is exactly right.
Block UV light if you can as well. If you can block from the outside of the glass, even better. That's why shutters work so well. Fabric only has limited UV blocking capacity which is why closing curtains doesn't always help, but it's still better than nothing.
Fans don't cool rooms even with ice under them. Turn them off unless they are actually cooling a person.
Turn off anything electrical that you don't absolutely need as they give off heat. Avoid using oven or grill or boiling things. Avoid tumble dryer use.
Bodies are hot as well. Don't all congregate in the same room unless you have to. However if you have a young baby, skin to skin contact will help regulate their temperature. Young children may do this too but unfortunately for them it's very annoying.
Along these lines, if you can go out safely, go out. Find somewhere with shade, water, trees or higher altitude, or somewhere like a shopping centre, museum or cinema might have air con. Avoid town centres and anywhere with little shade or too much concrete.
If you have a portable air con unit, don't wait for it to get hot, start it when it's comfortable in the room you're spending most time in and it should keep it comfortable much more easily than if you're trying to make it cool a hot room down.
Tepid (not cold) showers and baths will help you. Also freeze bottles of water and drink from the frozen bottle as it defrosts.
This is what I have learned from 9 summers in a country that is sightly hotter than the UK but built for cold. We usually get 30+ in summer, very rarely up to 40. The first 3 years we lived right under a roof in basically a single room as well so summers were really hard. We used to go and jump in the stream or just go for a drive in the air conditioned car to get away!