Usually when you visit St Paul's Cathedral you are not allowed to take a camera of any kind, other than a phone and even then you aren't supposed to be taking pictures with it. They do however have the odd occasion where they welcome people with their cameras. The Reformation Lates is one of these occasions (you still aren't allowed to take pictures in the whispering gallery).
In an ideal world you would use a tripod to stabilise your camera as the light levels are pretty low. Sadly they were not allowed at the Lates due to health and safety, although I did see one guy with one, he did seem to know the staff,so may have had special permission.
Not to be put off by this, I instead took a beanbag to rest my camera on and used other ways to stabilise it. Like laying it on its back and pointing it up. When I got fed up with that I just put on a fast prime or upped my ISO.