Having studied this problem at length, I can proffer a solution to the gentleman's problem. This provides a contingency in the event that the CIS does not display the location of first class, or that it is to be distrusted.
Almost all TL 8 car services that have any first class are formed of 2 units each with a first class compartment. Each of these first class compartments can only be at the front or back of their respective four car unit.
Therefore, stand at the point of the platform precisely in the mid point of either of the 4 car units. As the front coach of that unit comes into view, check for the first class marker by the door (visible from a good 20m away) to determine whether it is at the front or back of that unit. Once determined, walk the 35 metres to the appropriate door, which should take no longer than 20 seconds, or approximately the same time it takes for the train to come to a stand, the doors to be released and opened. Board the train, raising an eyebrow of superiority at your fellow commuters as you enter the sanctity of the first class compartment. For additional enjoyment, ask anyone in there to produce their first class ticket, especially if they have boarded at Elstree & Borehamwood.