A couple of times recently I've attempted to buy a ticket from
Boundary Zone 2 to Cambridge (either CDR or SVR) from Finsbury Park
station ticket office, and was issued instead with a ticket Finsbury
Park to Cambridge, Route Not London.

The second time I saw what the (same) clerk was doing before he issued
the ticket and tried to argue the toss but got nowhere - he insisted
on issuing the ticket from FPK "because it's the last station in Zone
2". The problem is that these tickets are *almost*but*not*quite*
equivalent (provided your using a 7 day or longer Travelcard). The
prices are the same, the restriction codes are the same and in
particular, AIUI the train doesn't have to stop at FPK, by virtue of
the rule about using one ticket that is a season ticket and one ticket
that is not (7 day Travelcards and longer are considered season
tickets for the purpose of this rule).

But, and there's a big but -- the ticket I requested is interavailable
between First Capital Connect and National Express East Anglia - the
one sold to me is not (because the West Anglia Main Line is a
permitted route between London Terminals and Cambridge, but it's not a
permitted route between Finsbury Park and Cambridge).

So it seems to me that FCC are deliberately refusing to sell me the
ticket I ask for because because there's another ticket that is in
identical in price and availability (this can't be an accident) but
ORCATS will allocate them a larger share of the ticket price
(presumably almost all - I think there may be the very odd journey
where change-at-Stevenage onto East Coast may be plausible).

The problem is that although you typically will want to use FCC on
that route, using NXEA is not entirely implausibe. It's particuarly
likely on a Sunday when fast trains are only hourly. If you just miss
the xx28 semi-fast train from CBG to KGX (which calls at FPK) then
there's a huge advantage in catching the NXEA service to LST four
minutes later, alighting at TOM and completing the journey by tube by
Oyster PAYG.

It's annoying enough that they insist you queue at the ticket office
to buy a Boundary Zone ticket, but when they refuse to sell you one
even there - what do you do? Well you rant on uk.transport.london and
uk.railway of course :-)

-roy