The child is a British citizen- please see this from the govt site.
//3. CHILDREN BORN OUTSIDE THE UNITED KINGDOM TO BRITISH
CITIZENS OTHERWISE THAN BY DESCENT
A child born outside the United Kingdom will be a British citizen by descent if
either parent is a British citizen otherwise than by descent at the time of the birth.
However, in certain cases, a child who is born to a British citizen otherwise than
by descent may also be a British citizen otherwise than by descent (see section 5
below).
4. CHILDREN WHO ARE BORN OUTSIDE THE UNITED KINGDOM TO
BRITISH CITIZENS BY DESCENT
A child born outside the United Kingdom will not be a British citizen if neither
parent is a British citizen otherwise than by descent. A parent who is a British
citizen by descent cannot normally pass that status on. The exception to this is
where the parent was in one of the 3 types of service listed in section 5 at the
time of the birth.
A child who is not a British citizen may be entitled to be registered as a British
citizen. Further information can be found on the Gov.Uk website:
https://www.gov.uk/register-british-citizen/children-born-outside-uk.//