epi wrote: ↑Fri Aug 02, 2019 5:11 pm
there will be no proof such as a hard copy certificate confirming that they are British?
Correct. Because they acquired British citizenship at birth, there is no hard copy certificate confirming their British citizenship. Such a certificate (of either registration or naturalisation) would only be issued if they acquired British citizenship at some point after birth.
However, they will need to keep their father's naturalisation certificate and their own birth certificate safe as that that is the only proof of their (and their children's) British citizenship.
Of course, as British citizens by descent, their children will only be British citizens automatically if their children (your friend's grandchildren) are born in the UK. Any children born to the twins outside the UK will not be born British citizens.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.