One loophole is if the guy in the left hand seat is a JAA FI then you technically could log it as dual instruction received. Although I personally wouldn't recommend it.
I would just make a note of the flights you do and raise the issue in any interviews you have in the future. The fact that you are giving your time to learn and watch how a light commercial aircraft operates from the front is great experience and should impress the interviewer.
Even post-PPL, although rated on the aircraft you could log it as PIC, you have to prove that you are paying pro-rata share of the costs as flying the 'dead legs' wouldn't be covered by a PPL. To legally be able to log the hours in this case, you'd need to hold a CPL and given the UK weather, a IR. Also the guy in the left seat wouldn't be able to log any time as the aircraft is a single pilot aircraft.
Hope this helps.