1. This is the bus to London.
2. This is the bus for London.
Both are correct. "To" focuses on the destination; "for" focuses on the purpose.
If a bus stops at X, Y, and Z, you would probably be slightly more likely to say "Is this the bus for Y?", if you intended to alight at Y, and slightly more likely to say "Is this the bus to Z?", if you wanted to know the final destination.
Student or Learner