Have you recently moved to this house? A massive power consumption
like this would be reflected in your electricity bills.

A possible explanation is that the wiring is very different to what
you think it is - and a heavy load of this sort is normal (e.g.
shower, cooker, storage heaters) - or the wiring is so mad that a
heavy current device is actually supplied through 2 different MCB's.

I would start by thoroughly mapping each circuit from the consumer
unit. All mcb's off - check every single socket, light and appliance
is without power (in case something has been wired before the mcb's!).
Check zero consumption on the meter. Then one mcb at once, verify
every single socket, light & appliance - would be a good time to use a
plug-in polarity & earth tester.

If the behaviour persists, and there's a massive current load on only
one socket ring with no plugged in loads, and the behaviour is
repeatable by switching that mcb with all others isolated - first
panic, then consider opening up sockets, disconnecting parts of the
ring etc.