Hi,
In the UK, tax and National Insurance contributions are separate although they are both collected by HMRC.
National Insurance contributions (NIC) are what counts towards entitlement to Social Security benefits including State Pension. If you are living and not working in the UK, you may be entitled to pay voluntary NIC.
You will need to be registered for UK National Insurance numbers if you are actively seeking employment or self-employment in the UK. You can see information about National Insurance in the UK here:
National Insurance: introduction
You should submit your Self Assessment returns separately.
Thank you.