UCL uses Turnitin, a plagiarism detection tool which checks students' work against a large database of sources such as journal articles, websites and other students’ work.
It provides a 'similarity index score' expressed as a percentage, which you can use as a guide for how much of your own text matches other sources. You can check your work in a test version via the Plagiarism and academic writing for students Moodle course. When you then submit your work as an assignment, it will not be compared against your test version in Turnitin.