Congrats and Good luck.
1. If you are near Hampstead, the Royal Free has a good reputation. I have had a few friends who have given birth there.
2. Yes. Register with a local GP. Tell them you are pregnant. Mine had me come in for an appt, but all they did was take my blood pressure and put me in the system to be contacted by the midwife team in your area. Some GP's will skip the office appt and just put you in the system for the midwives. By about week 12 a midwife will have visited you to take blood and do the initial pregnancy stuff. Then you should have regular appts with the team either at the hospital or at your GP surgery. Your ultrasounds will be around week 12 and week 20 if you are having a low risk pregnancy. They will be at the hospital.
3. The midwife teams are assigned by area, so the likelihood is that the same midwives will be responsible for all the GP surgeries in your area. The midwives you see for pre-natal care, tend not to be the ones you see in the delivery room at the hospital. You can tour the various birthing centers/hospitals in the area, but you don't tend to meet the staff that you'll be dealing with on the day of the delivery. It's more just to get the lay of the land and see the facilities.
4. A maternity ward is what you would think of as a standard hospital environment. They are prepared to give full surgical care should you require it during the birth, i.e C-section. A birthing centre is a more natural environment and is not equipped to deal with high risk pregnancies or emergency situations. They are also limited in the drugs they can administer for pain. You cannot get an epidural. Some birthing centres are standalone facilities, like Edgware Birth Centre and some are right in the same wing as the standard maternity wing, like Barnet General hospital...to name a few north/NW london hospitals. If you were looking to explore the birth centre option, the set up at Barnet is good. The birthing centre rooms are right down the hall from the standard rooms, so that if you did need medical intervention, it's just wheeling you down the hall, as opposed to Edgware...they would have to put you in an ambulance and transport you to Barnet. About a 15 minute drive. Just something to consider. I don't know the set up at the Royal Free. But the hospital websites should help out.
5. Check out prental classes through the NCT (National Childbirth Trust). It will cost about £300 but you will meet mums due around the same time in your area.