Freshers Week, and indeed the start of term starts on Monday 27th September.
Most students will move in the day before, Sunday 26th September.
You can usually move in before that date, as long as the Residence Office agrees, and you pay for it.
As you are an international student, you usually move in the week before home students, and follow the International Student Orientation Programme.
Considering it is only April, I doubt any of this has been organised. You will receive information from UCL about this though.
Might be worth emailing the International Office about what date you should book flights...