Why is it illegal to collect rain water?

@Caz82 dunno

@Caz82 Many people just use it to water a garden.

@Ferise1 I suppose you could use it for toilet/heating if you collected enough and had a decent sized tank.

Seems like it would still be a lot of hassle to save not much money.

@Caz82 Why would it need to be purified?

@Caz82 @MrBrownstone Rainwater is pure

@ArtieKat Not at point of use, it isn't.

As it falls it is basically distilled water, apart from naturally containing a tiny amount of dissolved atmospheric carbon-dioxide (which is harmless), and is clean. Although it also lacks minerals that make normal tap-water more useful, perhaps better, than distilled water to drink.

Once it's washed the roof and gutters, and collected without treatment in a tank very hard to clean to a safe standard, it is not clean and becomes rapidly stagnant, rich in harmful bacteria, unsafe to drink and likely smelling and tasting "off".

@MrBrownstone Asd it falls, it is pure. Once it's collected from a roof and stored for a while, it is not safe to drink.

@Caz82 The toilet yes, though as you say it would need a very large tank to store a useful amount for thet.

Heating, no.
Central-heating systems are closed circuits that normally should only need a very small topping-up about once a year. This requires a full connection to the mains because it needs a modest but appreciable pressure to inject the water.