Like most I visited St Paul's and it's quite a space, worth seeing. Rick is pretty high (pun intended) on going up into and onto the dome, I was standing near the entrance to it reading all the disclaimers (once you start you can't turn back, no claustrophobics, heart conditions, weak legs knees hips etc.). The woman guarding the stairway said So are you going to do it? I said I'm still thinking about it, she said Good! With great emphasis.
I found the upstairs and the downstairs more than enough, and you get a free audioguide. Nice gift shop on the way out and a small coffee cart with pastries.
Also be sure to look on the fencing outside the church for daily schedules, they'll tell you when there is evensong and when they might be closed for a special event. There was such a thing on one of my days in town, apparently some royals were involved in the ceremony based on the size of the cars in the entourage and the number of motorcycle cops. People were standing around outside asking each other if they knew who was there, no one did.