Epetruk wrote:
I usually make my daily commute from East Croydon to Harrow and Wealdstone
using a zone 2-5 travelcard via Southern Trains. However, from time to time,
I find (either because I've missed a train or I have to travel at a time
earlier/later than my usual time) that I have to go through zone 1. What I
would like is a ticket that I can buy in advance and hold so that I can make
payment without having to wait at the ticket counters.
...
Does anyone have any further suggestions?

Do you ever use the Silverlink Clapham - Willesden Junction trains?
They give you extra options if you take the first available train to
the first junction, though it'll always be slower than one through train.

I think they're half-hourly.

Colin McKenzie