The influence of magnetic field on the structural ordering in the concentrated ($\sim $ 30 g/l) aqueous colloids of strontium hexaferrite (SrFe$_{12}$O$_{19}$) nanoplates with an average diameter of 500 \AA and a thickness of 50 \AA were studied with polarized small-angle polarized neutron scattering (SANS). Two-dimensional SANS intensity maps illustrate isotropic diffuse scattering in the absence of a magnetic field and a series of diffraction reflections arising along the field at Q $\approx $ 0.0185 \AA$^{-1}$ and its multiples. The appearance of Bragg maxima with the magnetic field is attributed to the formation of columns (chains) of flat strontium hexaferrite nanoplatelets distanced at $\sim $ 340 \AA. According to the dependence of the Bragg peak intensity on the magnetic field, three structural states of the ferrofluid were identified during magnetization process. At H $\approx $ 0, individual platelets or segments of uncorrelated platelet chains are randomly oriented in the isotropic phase. At H $>$ 20 Oe the platelets orient preferably along the magnetic field and assemble into staked plate columns. In the intermediate state, 20 Oe $<$ H $<$ 250 Oe, these staked columns exhibit 1D-nematic order. At H $>$ 250 Oe the columns order themselves, presumably into a hexagonal smectic discotic structure with a period exceeding 750 \AA. The amplitude of nuclear magnetic interference scattering also reveals a stepwise increase with amplification of the magnetic field at H $\sim $ 20 Oe and H $\sim $ 250 Oe, showing the correlation of the magnetic and the structural ordering and collinear magnetization in the neighboring columns within the magnetized ferrofluid.