Martin Petrov wrote

I'm flying out of Gatwick tonight at 6pm, and am wary that if I turn
up at London Bridge this evening at about 4pm, I might find that
there's a queue a mile long at London Bridge ticket office - can I
buy
a ticket from my originating tube station that will take me all the
way through? Or am I stuck with having to queue up at London Bridge?

Are there any alternatives?

Lots.

You can buy rail tickets from many travel agents
or from most NR stations.
or over the internet for collection from a ticket machine at London
Bridge.

In these circumstance I usually bought in the morning or the night
before.

--
Mike D