I couldn't disagree with newname more. As a Londoner, I consider myself extremely lucky to be able to choose between multiple teaching hospitals (which have all been rated highly by CQC) that are affiliated with world class universities.
Not all of the hospitals will have a modern exterior but the majority have had extensive refurbishment within the past 10-15 years and all have modern equipment.
Being the NHS, all are extremely overstretched but the reputation of the hospitals mean they attract some of the best staff, and the overwhelming majority go above and beyond for their patients.
As a pp mentioned, the leading professor at Kings is linked to the Fetal Medicine Centre and Kings are very highly regarded for their antenatal care - considered the best in the country by a lot of healthcare professionals. They offer a growth scan to all women at around 36 weeks and also offer harmony test on NHS if you are considered to be high risk for Downs.
I am at UCH - also very highly regarded by most healthcare workers. I have found that I haven't had much time to discuss any concerns with the midwife before 20 weeks and information has been lacking slightly but I have every confidence the care as pregnancy progresses will be excellent. All staff are friendly and helpful. Only thing to note is they have removed the 32 week growth scan for low risk pregnancies now. As a separate note, the equipment is the same as that used in private centres.
St Thomas is also fantastic, all be it stretched on the postnatal unit. The midwives are great and equipment is modern.
I don't know as much about St Marys, Chelsea and Westminster and Queen Charlotte but I have only heard good things about all 3.
All of the above have absolutely fantastic neonatal care units (even though you hope you won't need one)
Check out each hospital's website to find out about options for private rooms if that is important to you. If you are paying for your care, I agree with a pp to pick an excellent NHS hospital and deliver in the private suite.
Hope that helps