I have seen homes in England with both gas heaters and electric on demand heater. I never understood it. It seems salespeople sell stuff and homeowners buy stuff they don't really need because they do not understand the problem or the system.
With an electric heater it is trivial to insert a triac power control and regulate the temperature manually but it is better to sense the temperature automatically as it leaves the heater and insert a small tank or long pipe to stabilize any fluctuations.
For a while I was staying in China in an apartment with a gas on-demand water heater which was impossible to get a decent temperature. The water was either scalding or boiling. I very soon learned to use two low stools. Put a tub on one and mix hot and cold water until I got the desired temperature. Then sit on the other stool and use a mug to shower water from the tub over myself. Works pretty well and saves a lot of water.
I was doing the same thing on my boat until I fixed the system which has its own complications, including cycling pumps.
Interesting comments. A few of them, from my experience...
Having an on-demand electric water heater is common in most UK households with a hot tank. It's simply because you might not want to put the heat on in summer. Might be a surprise but none of the houses I have lived with a hot tank have had the ability to select "Hot water only" for the boiler. So in summer you simply wouldn't use the heating to get hot water as that would heat the house radiators too.
The on-demand heater, usually referred to locally as "The immersion heater" and often a switch or pull cord in the kitchen is a 13A or 30A immersion heater. However it's not just an element stuck at the bottom of the hot tank. It takes water out of the bottom, heats it through a coil and via convection dumps it into the top of the tank where it will stay, heating the water from the top down. The hotwater outlet is designed to take water from the top of the tank if available. In my house if the water is cold because the heating has been off, the "immersion heater" will have it hot enough to scald your hands in about 1 minute. Obviously if you try and run a bath with that or have a shower it will use up the layer of hot water rapidly and it will run cold. You can heat the whole tank with the immersion heater but it would be wasteful.
This issue is why people put electric showers in. It's so that they don't need to rely on running the heating in summer or using the immersion.
Yes systems do exist with an electrical operated valve to select "Hot water", "Radiators" or "Both" but they are very rare here.
A triac on a 7Kw water heater will never get past UK electrical codes. I expect it will be, large, noisy, expensive and likely to melt or go on fire at some point.
Hot water in the UK comes out of the tap as hot as you can legally make it. The 65*C target is very often breached by gas boilers and electric water heaters. My previous apartment would happily produce water close to 80*C. You could almost make tea or coffee with it. All most all taps in UK homes are mixer taps. This is a rather recent thing as there are regulations stating that the hot and cold taps be separate anywhere the hot tank is coming from a header tank system as it can potentially contain stagnant water. This regulation has become lax however and many people fit mixer taps in the bathroom regardless of the regs.
So you can expect the hot water to be absolutely scalding and expect to have to mix cold with it to get a temperature for bathing.