I presume the point of the apparently unenforceable restriction is purely in the interest of pax travelling beyond Reading who otherwise have to put up with Reading commuters crowding them out?
If so, why not just erect a sign at the platform entrance (and announce it both on train and station) before departure that pax to Reading will be charged ^10 extra as supplement. Agreed such pax would have to believe that a TTI would be walking through, which would be done occasionally. Actually it's only FO anyway isn't it? .
That procedure should clear the decks, I suggest.