What will you do when you are in London?
–>I’ll go to see my family

Notice that “when” is referring to a future time, but in English is always followed by a present verb. The following verb then is future simple – “will”

When I have finished(present perfect), I will eat.

For French speakers, this is something to be careful about, as translating word-for-word will result in unnatural sounding sentences like:
*When I will be in Paris, I will buy some new clothes