The parent has to be a UK citizen OR settled here, by which I imagine is meant that they have indefinite leave to remain here and a permanent home here. Show this to your solicitor:
Section 1(1) British Nationality Act 1981
3.2.2 Under s.1(1), a person born in the United Kingdom on or after 1 January 1983 is a British citizen at birth if, at the time of the birth:
* either parent is a British citizen; or
* either parent is settled in the United Kingdom