I have a monthly zone 2-3 travelcard on my Oyster but I'd like to pass
through zone 1 occasionally, via the mainline terminals at London Bridge
and Liverpool Street, without paying twice for the rest of my journey.

I think this will become asier once pre-pay is valid on NR but meanwhile
are there extension zone tickets available or else what's the best way?

I already discovered that a single ticket from LivP to Stratford is
£3.00 , but with pre-pay I'd have to jump off the train, swipe, and
jump back on again. That seems a bit crazy, so I'm stumped as to what's
the best thing to do. Any suggestions?