It's going to hit me personally - and I can see a lot of PFs handed out to unhappy customers over the next few weeks/months.
The same with the removal of 79, which was very useful to users of London suburban stations when connecting to/from EC services. Methinks it's time to complain to ATOC/DfT/London TravelWatch/anyone who will listen...
A PF would not be issuable in most such cases, see:
"13. The route you are entitled to take
(a) You may travel between the stations shown on the ticket you hold in:
(i) a through train;
(ii) trains which take the shortest route which can be used by scheduled
passenger services; or
(iii) trains which take the routes shown in the National Routeing Guide
(details as to how you can obtain this information will be available
when you buy your ticket).
(b) If you are using a Zonal Ticket you may travel in trains which take any route within
the zone or zones shown on the ticket.
...
(e) If you make your journey by a route other than those referred to in (a) and (b)
above, you will be liable to pay an excess fare. This excess fare will be the
difference between the price paid for the ticket you hold and the price of the
lowest priced ticket(s) available for immediate travel that would have entitled you
to travel by that route."
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
If a passenger with a Finsbury Park-Cambridge ticket travels via Kings Cross, in my opinion they should be excess to the tune of the diffeence between the fare paid and the sum of the appropriate Finsbury Park-Kings Cross and Kings Cross-Cambridge tickets.
The only way that I can see a PF being justified is if a passenger with such a ticket attempts to leave the station completely at Kings Cross, in which case it is debateable.