MOO JMO – Occasional WS visitor, first-time poster.
I don't know if it's been officially confirmed anywhere that what was caught on the bus camera was WC's car, maybe it was him following SE on foot?
It's been stated that SE's call to her BF was normal, ended normally and her BF wasn't worried until the next evening, when they were supposed to meet and he didn't hear from her. This was when the alarm was raised that SE was missing, I believe. As it's been theorised by other posters on this thread, maybe WC rented a parking space on Poynder's Road in order to avoid central London/congestion charge etc. and get home to Deal quicker. This would be his link with the area and reason for being there after his work shift ended.
WC exits the underground at Clapham South and follows the same route as SE to get to his parked car, walking behind SE as she's talking to her BF on the phone. She talks about their plans the following evening and what she's doing the next day (working from home, presumably). WC can hear her side of the conversation and now knows:
1. She isn't on her way to meet her BF/anyone, just heading home to her own flat
2. If she goes missing now, this won't become obvious until the following evening
SE ends the call in a normal manner, her BF isn't alarmed. WC is right behind her and approaching where his own car is parked. He seizes the opportunity to bring her under his control (element of surprise, strength, and training in getting people quickly out of dangerous situations are on his side) and into his car.
Maybe this is why the doorbell footage hasn't been publicised – because WC is also visible walking close to SE. This also explains how the timing is so perfect – SE ends her call normally, BF doesn't think anything is up, two minutes later the phone is off the grid and SE is gone.
TLDR: WC happened to be behind SE on foot, walking from the Underground to where his car was parked at Poynder's Court – listening to her phone call. From listening to her call, he knew the alarm wouldn't be raised right away and his car was close by, so he took a chance (possible reason: knew he was going to get caught for IE incident a few days earlier or escalating behaviour).