There are actually three varieties:

1. Mixer tap, with one handle
2. Mixer tap, with separate hot and cold handles
3. Two completely separate taps, such that the hot and cold water only mix in the sink

Obviously, Type 1 is most convenient.

Type 2 does have an advantage in energy savings. Often, with a single handle, a user will leave the handle somewhere in the middle, such that it draws water from both the hot and cold pipes. However, the hot-water draw is often unintentional. The user will just wash his/her hands for a few seconds, not long enough for the water to get warm, and not really caring that the water is cold. Nevertheless, drawing cooled water from the hot side does have a consequence, since the hot water tank will need refilling. Therefore, users of single-handle mixer taps will often unknowingly waste energy heating water for absolutely no benefit.

The British, though, seem to be stuck on Type 3. Part of the reason is a fear that a dirty heater tank could contaminate the cold water supply if mixing were allowed. Mostly, it seems to be a case of stubborn conservatism.

There are actually three varieties:

1. Mixer tap, with one handle
2. Mixer tap, with separate hot and cold handles
3. Two completely separate taps, such that the hot and cold water only mix in the sink

Obviously, Type 1 is most convenient.

Type 2 does have an advantage in energy savings. Often, with a single handle, a user will leave the handle somewhere in the middle, such that it draws water from both the hot and cold pipes. However, the hot-water draw is often unintentional. The user will just wash his/her hands for a few seconds, not long enough for the water to get warm, and not really caring that the water is cold. Nevertheless, drawing cooled water from the hot side does have a consequence, since the hot water tank will need refilling. Therefore, users of single-handle mixer taps will often unknowingly waste energy heating water for absolutely no benefit. (The Swiss government, for example, runs a campaign to remind people to keep the lever turned all the way to the right, which would not be necessary with a two-handle system.)

The British, though, seem to be stuck on Type 3. Part of the reason is a fear that a dirty heater tank could contaminate the cold water supply if mixing were allowed. Mostly, it seems to be a case of stubborn conservatism.