(...dropping by here on request...)
closet is right.
US and UK law are similar in lots of ways *EXCEPT* about citizenship. The US system is called "jus soli", it means "by right of the soil". And the UK system is something else. So the intuitive notions that an American would have about a baby's citizenship don't work here.
Historically, it helps to remember that the UK once owned about 1/6 of the world, so the "jus soli" system was not practical to administer. And they began curbing it back in Victorian times, and only the distaff side could pass on citizenship under certain circumstances.
As closet says, the final nail in the coffin came in 1981. And it's all nailed down very tightly.
A baby born in the UK has no automatic right to British citizenship full stop.However, buried deep in the legal mumbo jumbo there are two loopholes. If a baby is found on UK soil abandonned, and they cannot find the parents, the baby receives citizenship by otherwise than descent regardless of its origin.
And the second is even *more* obscure: If a baby is born in international waters on a British carrier and it has British blood from either parent *and* it would otherwise be stateless, the baby also gets citizenship by otherwise than descent. I wouldn't recommend trying either of these tho'.