K1119's comment above sounds like very sensible advice to me!
For what it's worth for other's future reference, we had a great experience at Thomas's. Much of it with NHS hospitals is surely down to luck - ie: how busy it is when you happen to go into labour?
Closeness and convenience for D-Day (i.e. Thomas's was nearest) made me a little less stressed - which down to tight timeframes on the night ended up being important!
NCT friends of mine ended up in Chelsea on a crowded ward, I had a room to myself directly opposite Big Ben for free
I'm sure I just got lucky but there are admin nightmares to varying degrees across the system overall, i didn't notice St Thomas worse than others (and I had a many pregnancy issues and appts)