I am not familiar with UK practices, but I don’t the there will be a significant heating problem if the neutral is the same size as the phase conductors in branch circuits. Discharge lighting {fluorescent, mercury, sodium} does have the capability of producing triplen current harmonics in the neutral of wye circuits where feeding phase-to-neutral loads, but it seems to be overrated in a lot of cases. It wouldn’t hurt to check things with a clip-on ammeter with the finished installation. [There are oversized-neutral multiconductor cables available in the US, but I don’t think they are specified or even marketed for lighting.]
I agree that using 3ø to minimize flicker problems with discharge lighting to eliminate strobe effects [perception of ‘freezing of observed motion’] over rotating machinery is good to consider.
