I can't see a problem to this if the tank is against the wall and the floor
joists are 8x2" or bigger and the joists are at right angles to the tank
long dimension. A cast iron bath is in the region of 200Kg and full of water
is another 200 Kg, then add another 200Kg for the people standing beside it,
I've not seen many bathroom floors bend much under these sort of loadings.
The loading figures you quote are for the complete floor area, you are
really talking about point loading on a floor area. A lot will depend on
the actual unsupported span of the floor joists and the position of the tank
relative to the supporting walls. The building regs gives a good guide to
the actual loading a given joist and span will support. Can you tell us what
the joists and spans actually are?
Interesting query.

Regards
Capitol