Observation: it's summer again.
Observation: the London Underground is the oldest, hottest, most poorly-air-conditioned underground mass transit system in the world. It is an engineering impossibility to bring it up to the standards of the 21st century. London itself is pretty hot too. I guess other cities would benefit from this idea too, but London is obviously the place to test it.
Plan: buy bottled water. Freeze the bottles. Sell them on the street.
Water has a high specific heat capacity which has two direct results. Firstly, freezing a bottle of water requires a large quantity of energy which means it will cost money. Secondly, a bottle of ice stays frozen for a long period of time and takes a long time to melt. Even if you can't drink the ice immediately, you can still cool yourself off by holding against your forehead or wrists.
You could either have a freezer with you on the street, which would require power, or freeze the bottles at a central location and distribute them insulated cold boxes. All you'd need is a day's notice of the heatwave and you could freeze the water overnight.
Since the water is just regular bottled water, unopened, you have no specific requirements to ensure the safety and quality of the water, and the customer gets peace of mind. And you can charge a premium.