In this Sky News article it says Petra withdrew money from a cash point in the Elephant and Castle area. This means she could have got on the C10 for convenience as it stops outside the hospital. Got off at Elephant and Castle, withdrew money, then got on a bus to Camberwell.
Petra Srncova: Missing woman from south London last seen on bus after withdrawing cash, police say
From what I've noticed a lot of the articles covering this case can't seem to agree on the exact moment when (and place where) she used the ATM to withdraw the money she needed. Some state she first got the cash then got on the bus, others say that it was the other way around and she actually withdrew the money once she got to Elephant and Castle. Either way I personally believe that the C10 would be our safest bet.
ETA: Accounts seem to differ bewhen it comes to the last sighting of Petra as well. Most articles state that this was at 8:22 PM but are a bit vague when it comes to whether this was on the bus or in and around the Camberwell area whilst others are adamant it was the latter.