The transition line between ice VII and ice VIII has been determined by Raman scattering for H2O and D2O. Above 15 GPa the transition temperature decreases linearly with pressure, and then drops to 0 K around 62 and 72 GPa for H2O and D2O, respectively. An analysis, using the pseudospin formalism, shows that the linear regime is due to the decrease with pressure of the dipolar interaction, while the proton tunnelling remains negligible. The proton disorder becomes efficient only from 60 GPa for H2O (or 72 for D2O) where it limits the stability range of the ordered phase VIII. Our analysis suggests that ice X, the so-called ice with hydrogen at the midpoint of the O---O bonds, would be obtained above 80 GPa (H2O) or 100 GPa (D2O).