The top switch is called a fused connection unit and is most likely the supply to immersion heater, the next is a simple switch and I have really no idea what it might be for, the next box is likely the connection to central heating and the unit held to the tank is likely the sensor for the central heating so it knows when to heat the hot water.
Domestic most switches go down for on, not domestic is the reverse so anything falling on a switch will switch it off not on.
The immersion heater should have a thermostat built in, and also a cut out, if the tank is thermal plastic it should be non resetable but if the tank is hard plastic or steel then a resetable type can be used.
The thermostat in the immersion heater should be set above 66 degs C but less than the central heating so it will only cut in if central heating is not running.
To find out what you have some on site investigation is needed. By the pictures we can only assume you have a fused connection unit protecting some part of the wiring nearby. If you could track the white cable heading off behind the cylinder, then we can determine what that is doing. I do hope it is an isolation switch, rather than a standard light switch as they are rated differently.And it could be under-rated.
Also, I would have thought that when i had both switches pressed in at the top then the immersion would be on, as per ericmark's comments about non domestic switches being the reverse of a domestic switch.
As you can see from the attached photo, 'off' is written at the top. Does this mean that the switch is off when the switch sticks out (pushed down), rather than pushed in at the top?
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