I was in Twickenham recently, and could not find anywhere that could
top up an Oyster. If you work near somewhere like Oxford Circus, you
are lucky to get into the station in the evening, let alone get near
the ticket counter, if you remember.

So do it at Oxford Circus in the morning when there's no queue. It's
easier to use the machines than the booth, anyway - just click the
"renew existing season ticket" button and it takes about 30 seconds.

In any case, one of the effects of Oyster is that the queues for the
ticket office are now much less. On a couple of recent occasions
when I have had to use the ticket counter at Green Park, I've found
hardly any queue at lunchtime - a big change over the last year or so.