In message , neverwas
writes
I can see that the grippers wouldn't necessarily know I was not getting
off *in* Zone 1 and touching out there.

Isn't it much the same as if you cross London with a NR ticket from one
side of London to another with a + on it: you can cross zone 1 on the
tube but cannot validly get out except at designated NR terminus
stations?

On the other hand they could if
were gripped while transiting from zone 1 to zone 2.

Even so, you would be travelling on a valid Oyster ticket, having
touched in at the start; they wouldn't know where you are planning to
touch out, would they? Is there any requirement to re-touch en-route
to show the Oyster system that you are using a particular route (except
if it is to your advantage to show that you are using a cheaper route
such as one which avoids zone 1)? I don't think so.

It seems to me that such a journey is valid at all points, and if the
system charges you a cheaper fare on the assumption that you have used a
particular route, that is not your problem.

--
Clive Page