I need to get from Mile End to Shalford, and I don't happen to have a train ticket already. I believe I can take underground to Whitechapel, then overground to Clapham Junction (avoiding zone 1) and then take a train to Guildford. The question is: what should I do at Clapham Junction changing from overground to Southern? I use Oyster within London. Do I need to exit at Clapham Junction and re-enter? Or is there another way to terminate my Oyster journey?
If you want to use Oyster for the London portion of your journey, you will either need to tap out at one of the gatelines at Clapham Junction (CLJ) and then re-enter with your onward ticket, or you will need to tap out at the yellow reader at platform 17.
There are pink readers near the Overground platforms at CLJ, but these cannot be used to end a journey. They are intended to be touched if you are changing and continuing your journey using Oyster/contactless, to show you avoided Zone 1. In reality they can be used to
start a journey, but that is not an advertised feature and it doesn't work to
finish a journey.
If you're getting to Shalford via Guildford then you'll likely be departing CLJ from platform 9 (or 11 if it's a semifast or stopper). If you're going via Redhill on the Reigate service then it'll be from platform 13 (or occasionally 15). So it's a bit more convenient to use the platform 17 yellow reader in the latter case.