The equipment for this costs tens of pounds, rather than the thousands needed for a more complicated system. A complete water butt kit will include a diverter pipe to fit to your down-pipe and prevent overflowing, and a stand to allow easy access to the tap.
If you can collect more rainwater than you can use in the garden, then you could consider using rainwater within the house for non-potable uses, such as flushing the toilet or in the washing machine.
The cost and environmental impact of installing and running (e.g. the pumps needed) a system to divery rainwater for toilet flushing can be quite high, and might outweigh the environmental and financial benefits, so it is worth looking carefully at the options. For more, see: Can I flush my toilet with rainwater?
The vast majority of the carbon emissions related to domestic water use are from the energy needed to heat water in the home, so the first measure should always be to minimise hot water use in particular, for example with a water-efficient shower head and taps. Look also at overall water efficiency measures, such as a low-flush toilet. For more advice on water saving measures, see: How can I conserve water?
In other countries, the comparison between rainwater collection and mains water is often different. For example, in Australia mains water is not widespread and there is a much more acute water shortage, while in Germany the principal source of mains water is from groundwater, which in their case is very dirty and requires expensive treatment.