Congratulations on your pregnancy!
I live near Watford and gave birth to my first child there almost 2 years ago, and many of my friends have given birth there over the last 6 or so years. The idea was to go to the Birth Centre but since DD ended up being almost three weeks overdue, that didn't happen! I had an induction and the full works, which I wouldn't say was pleasant but all turned out fine.
So, my personal experience, which is of course only that, was:
The pre-natal ward (where they put you if you're on the way but not yet ready to go up to the delivery suite, or awaiting an induction) is pretty horrible - very little attention from midwives, lots of beds to a room, etc. I was kept there for three days waiting an induction. I now think that if I had kicked up a fuss they would have sent me home and called me back when there was actually room in the delivery suite for me, but at the time we didn't know that. Ridiculous as it sounds, I actually thought that if I discharged myself, they would wash their hands of me and no one would deliver my baby! That was the worst bit - being told several times a day that I would be induced shortly, and then 9 PM coming round and my partner being told to go home as nothing was going to happen. I'm really not sure why they kept me there using up resources!
However, the quality of care in the actual delivery suite while giving birth was fantastic and I can't fault them in any way. Everything was explained to us brilliantly and we never had a moment of worry that they weren't doing everything they could as quickly as they could to get the baby out safely. Midwives and doctors were all lovely to us. Post giving birth, they pretty much abandoned us (without cleaning up the room that was by then very yucky) and I still think they actually forgot about us for a while!
The post-natal ward was crowded and very busy and quite short-staffed (it was over New Year), and although none of the staff were unpleasant to me - or anyone else that I saw - they weren't really able to help with establishing breastfeeding for example. There was one fantastic male midwife, but the rest passed me by in a bit of a blur. (Fortunately the community midwives in my town are amazing and I got lots of help after we were discharged.) My advice on the post-natal ward is that if you want something you need to go and bother them about it several times. I suppose this is par for course in busy maternity units.
DC2 is due in two weeks and I am once again hoping to go to the birth centre at Watford, where everyone I know who has been there has had a fantastic experience. And if it ends up being in the standard maternity unit, I don't really mind.
Of all the people I know who have given birth at Watford (off the top of my head, probably about 15 births), only two have had terrible experiences.
And I know two people who have given birth at UCLH (London residents both) and have had great experiences there.
Watford do tours of both the birth centre and the main hospital bit, once you're a bit further along. That might help you decide.
Apologies for the information overload!
Good luck!