Most kitchen taps are supplied directly from the mains. As it is illegal to mix cold water from the mains supply and hot water from a storage tank in the same pipe, kitchen mixer taps will have separate channels in the spout for carrying the hot and cold water. Kitchen taps are designed to swivel from side to side enabling them to reach a double sink.
As bath taps are usually supplied from storage tanks it is acceptable for the hot and cold water to mix within the body of the tap. If the cold water comes direct from the mains supply, however, this type of mixer tap would be illegal. This is because a change in the mains water pressure can cause water from a storage tank to be sucked back into the mains, possibly contaminating drinking water supplies.