Confuse about questions structure

Question

I came across a sentence bellow

At what time does the next plane to London leave?

And I couldn't distinguish the difference from this statement

At what time does the next plane leave to London?

Which one is more correct or both are the same?

Answer 1

The first one is more correct. When in doubt, try to split the sentences into parts:

At what time does [the next plane to London] leave?

The subject in this sentence is "the next plane to London" - which is a perfectly correct way to distinguish it from both "the next plane to Madrid" or "the last plane to London". It's clear you're asking about the first plane which leaves for London, skipping all the other planes which leave for different destinations. On the other hand:

At what time does [the next plane] leave (to London)?

The subject is "the next plane" - without any other qualifier, it would mean the first departing plane, without any concern for the destination. And you're asking when that particular plane leaves to London - to which the answer might be "it doesn't", if the plane is going somewhere else.

Answer 2

I agree with @maciej-stachowski that the first version is better, and I agree with their analysis of that version's syntax. the next plane to London is what you're asking about, and it's the plane that is flying to London soonest from your location.

I also think that answer makes a good point that the next plane by itself would refer to whichever plane is going to leave the soonest (regardless of destination). The phrasing of your second version with to London at the end is awkward and doesn't sound quite right. However, I think that if you replace to with for you will get a valid question that is equivalent in meaning to your first version:

At what time does the next plane leave for London?

Normally, a vehicle (or person, or whatever) leaves for a particular destination (though they go to a destination, and maybe the word going is implicit in the phrase the next plane to London => the next plane [going] to London). I think you can ask the question this way and it will be understood that you're only asking about planes that are supposed to fly to London. That being said, I think a simpler way to phrase this question would be:

When's the next plane to London?

If you were to ask this, it would be understood that you were talking about the departure time (as opposed to the arrival time).