The paper is devoted to a general study on the comparison between the theory and polarimetric measurements on a selected dataset of JET discharges. The main aim of this study is to validate the ability of the model to predict the line-integrated plasma density determined using the Cotton-Mouton effect at high plasma density and temperature, i.e. at ITER relevant plasma parameters. The extrapolation of capability of polarimetry to conditions of a burning plasma can be carried out using the theory checked on the JET database; this assessment could be very valuable for the project of the ITER polarimeter. The rigorous numerical solution of the Stokes propagation equations (using dielectric tensor evaluated from equilibrium and Thomson scattering, including small correction terms due to finite electron temperature) of the laser beam inside the plasma are compared with the data available at JET from the polarimeter and interferometer diagnostics. The evaluated phase shift and Faraday rotation angle of the emerging beam for the considered chords of the JET polarimeter are compared with the corresponding measured quantities. The agreement with theory is satisfactory within the limits of experimental errors.The mutual action between Cotton-Mouton phase shift and Faraday rotation is a particularly important characteristic of the measurements at high density (and temperatures). In this context a novel improved analytical solution to the Stokes equations for the propagation of a polarized beam in a tokamak plasma is presented based on the approximation, checked on experimental data, that the radial component of the magnetic field can be neglected. The improved analytical solution is also compared with the exact numerical solution to propagation of polarization and good agreement is found. The comparison is carried out with other approximate solutions recently developed. Since the improved analytical solution originates from a clear physical approximation, it could contribute to clarify the mutual action between Cotton-Mouton phase shift and Faraday rotation.