The smaller tails aren't long enough to matter. Max permitted length of tails is 3m; 16mm² drop 2.8mV/A/m so three metres at 30A charging current is 0.25V; 25mm² drop 1.8V/A/m so 0.16V under the same worst-case conditions. 0.09V difference between the two. The supply cable could well be on the small side, but UKPN aren't proposing to do anything about that - they obviously believe that the voltage will still be within limits at the higher current if they just change the fuse. The voltage readings posted above seem to suggest the relatively low voltage is down to general load in the area rather than in this one house - it seemed to be fluctuating independent of the load drawn by the car. It would be interesting to do the experiment of winding the charge current up and down with the car's controls and seeing how much effect it has on the voltage.