This is mainly because the Convention Relating to the Status of Stateless Persons requires it.
Of course, as a soverign, the UK is free to ignore the convention, but being able to use it to deal with the nationals of other countries is more valuable than the theoretical ability to eject (whence to?) undesired birthright-citizens.
Of course, as a soverign, the UK is free to ignore the convention, but being able to use it to deal with the nationals of other countries is more valuable than the theoretical ability to eject (whence to?) undesired birthright-citizens.
https://en.wikipedia.org/wiki/Convention_Relating_to_the_Sta...