Free standing, catering water heaters, and thermostatically controlled boilers for home brewing may be of use if you only require 4-5 gallons of water that is not under pressure.
My brewing boiler has a tap near the base, and a separate thermostat switching the mains power on and off for whatever temperature I desire.
You just need to bucket in some water, and plug into a 13 amp socket.
Check the total electrical load will be safe for all the equipment that will be switched on at the same time.
My own experiences of colour developing was using a Paterson Thermodrum.
Washing only required a few jugs of pre-tempered water.
So it was not a big problem for me at the time.
Explore premium brewing equipment and boilers at The Home Brew Shop UK, perfect for every home brew enthusiast.
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Edit. Some of the boilers shown are meant to be used on stove tops or gas rings.
Plenty of small catering boilers out there.