As per ESMA's regulation - MiFID II that came into effect on January 3, 2018, regulated and reliable brokers such as Trading 212, need to confirm your nationality if you want to trade shares or other stock market listed instruments.
For example, solely UK nationals need to confirm their nationality by providing their National Insurance Number (NiN). If you have both British and Dutch nationality, we require both your UK National Insurance number and your Netherlands National Passport Number or a National Identity Card number (if you don't have a National Passport)