Indeed. If the ticket said 'London terminals' rather than St Pancras, then surely they fulfilled the terms of the ticket in that they travelled to 'London terminals' together?
The fact is, a heavy handed prosecution in this case means the two are more likely to just get a car next time in order to avoid any more hassle on the railway, thus the railway loses far more than the £2 or so they feel they have been 'defrauded'.
Edit: Whoops, didn't realise there was another two pages- apologies if this point has already been made