I'm in England next week is perfectly grammatical. It relies upon "next week" to express the idea of the future while using present tense. I believe that is what you had in mind when referring to what can be said in Spanish.
But we can express the future in another way too.
We can say
I am to be in England next week.
I will|shall be in England next week.
I am going to be in England next week.
Let's work backwards. The sentence with "am going to be" has a finite BE and a non-finite BE.
to be in England refers to a non-finite (i.e. outside of chronological time) existential/locative state, namely (being)-there-in-England.
Now, put going in front of the above, and you project that non-finite existential/locative state into the future, since going + infinitive places the action or state of the verb in the future:
going to be in England, namely, in-future-(being)-there-in-England.
Now, put I am in front of that, and you complement the bald existential statement "I am" with the existential/locative "in-future-(being)-there-in-England".
In other words, your current intention is to be in England next week.