This is such an interesting bit of architecture!
I love architecture, and when I am in London I shoot LOADS of it. The city has such a great collection of iconic sites, and I intend to shoot as many as I can!
London's City Hall is one of those sites, and of course you have that well-known bridge hanging out in the background too (that's the Tower Bridge, if you weren't aware). This part of London is great fun to walk around and explore.
And I definitely recommend walking as much as possible. While it's quick and easy to take the Tube across town (it's a large city, after all), whenever possible I think you should do some walking between sites. You are bound to see things that you would just completely miss if all you did was hop on the Tube from site to site. Plus you get in some exercise too. See? Photography is GOOD FOR YOU!