All giffgaff phone plans have a duration of one month, after which you can decide whether to activate another monthly plan or to discontinue it. The SIM will be deactivated if you do not top it up for 12 months. You will receive an e-mail shortly before the deadline reminding you to add some credit.
If you are new to the UK and want to work or study, you NEED an English phone number. It is essential to be perfectly integrated in your new country.