Please, forgive me, but I don't understand what you want to know exactly. In a previous thread, you asked a similar question:
I replied: In formal exams the expected answer is to use 'will'.
You asked: Why not 'is travelling'?
I replied: If you want to ask about the possibility of using the present progressive in the main clause, then the answer is 'yes'.
- When he reaches London, he will be staying / is going to stay / is staying with his friends.
Now, you ask the same question and David replied that both are correct. Strangely, you ask: Does it mean that we should use future simple not present continuous in the main clause?!!