Not necessarily so, child is still a Brit because parents are. Especially if born in flight over a body of water which belongs to no country. Don’t think air space counts. If any thing dual citizenship would be issued.
Step Daughter was born in Japan in a US Navy hospital, she had a passport at 3 days old, a BC from the hospital, from Japan and US Embassy. It gave her dual citizenship until she reached her majority of 18. Both parents were US Citizens. At 18 she had to decide which country she wanted, wise girl she chose the USA.