There are a number of places where you can look for internships in UK startups. If you are a student, you can find internship opportunities at your University's career office. You can also use platforms like LinkedIn or Otta to find tech related opportunities in the UK. Another option is to identify startups where you'd like to work and to reach out to them directly. A startup is likely to respond favourably or recommend you to other startups that are hiring interns if they felt that your outreach was genuine and tailored to them.