How can I improve my English if I have no native speaker to practice with?

Language is not used in a vacuum. Your not standing talking to a brick wall. The is another person who you are talking to or who is reading what you wrote. That is your hoop. Does the other person understand what you were trying to say? If not, you missed the hoop so try again. Maybe, they understand but spot an error or two, you got a near miss. If you talk and they understand perfectly, then that's your dunk shot.

The other side of the conversation need not have perfect English either. They can be a fellow student. In the UK, when learning foreign language, a great deal of time is spent on pair work and group work with students practising conversations without the teachers input.

The fact that you have sufficient English to read and understand what I wrote above, tells me that you have sufficient skills in reading. Likewise your fellow students can read and correct your writing or speaking.