Why do we use the future with will when we say "I'll be in London next week" when things have already been planned, instead of "be going to"?

Question

I know there are several ways of using the Future Tense, one of them being "be going to". This is used for intention, something which has already been planned, for example:

We're going to visit my sister this summer.

Will is used for deciding something at the moment of speaking, eg. "I'll make some coffee." However, why do we use the future with will when we say "I'll be in London next week" or "I'll be in the office tomorrow" when these seem things which have already been planned and are intentional, so why not use "be going to"?

Answer 1

There is a lot of overlap between the "will" future and the "going to" future.

In many situations, both are possible and both are correct.

In the examples you give "I'm going to be in London next week" is correct. And so is "I'll be in London next week." The difference in meaning so small that it doesn't matter.

"Going to" tends to be used for future events that will occur as a result of current internal conditions (either as plans or not). That explains why "I'm going to be sick" is correct. It means "I feel ill now, and as a result..."

If something is a result of some externality (such as timetable that was set long ago) then "will" is more likely. This might explain why a speaker would use "I'll be in London".