Why has anything to do with 12v come into the topic.
The fault is with the mains power
All the talk about chargers and BCA units is not applicable.
What the OP needs to do is what I have already said, disconnect the water heater mains cable at its junction near the heater to completely isolate it, and then try the electrics again.
He has not said he has tried that yet.
The OP has said he has removed fuses, but these are 12v only, not 230v , which is controlled only by the MCB's and any other fuses in individual appliances, which the water heater usually has one in the master switch, if fitted, in addition to the MCB.
The only other possibility is a faulty MCB , especially if it is a F&S unit.
However, given what the OP has said, and more importantly NOT said, I strongly suggest getting a mobile engineer to look at it .
A wrong move with 230v can kill,,,,,,,,,,,,