Good luck with this one! I used to work right by the Mayors office at London Bridge, I once went up on the roof as I was on good terms with all the building management, took some pics but seriously I wasted it. I wish I'd have put a table and chairs up there and made a little meal for the girlfriend, would be romantic and an amazing view!
I really should have gone up there with a tripod at night too.
I do recall access was not easy, had to climb a long ladder and go through a hatch IIRC.