We have derived a new calibration of the MV(HB)-[Fe/H] relation by exploiting the large photometric database of old GCs in M31 in the HST archive. We collected BVI data for 48 old GCs in M31 and analysed them by applying the same methods and procedures to all objects. We obtained a set of homogeneous colour-magnitude diagrams (CMDs) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs. Reddening, metallicity, horizontal branch (HB) luminosity and distance were determined self-consistently for each cluster. There are three main results of this study: i) the relation MV(HB)=0.25(+/-0.02)[Fe/H]+0.89(+/-0.03), which is obtained from the above parameters and is calibrated on the distances of the template Galactic GCs; ii) the distance modulus to M31 of (m-M)_0_=24.42+/-0.06mag, that is the first determination of the distance to M31 based on the characteristics of its GC system, and is calibrated on Galactic GCs, iii) the distance to the Large Magellanic Cloud (LMC), which is estimated to be 18.54+/-0.07mag as a consequence of the previous results.