Sunday is the only day that we have to see St. Paul's. If we attended a worship service (probably the evening one), would we get a good sense of the cathedral? (I don't want this to sound sacreligious--although we are Catholic we have attended worship services in various churches/temples/once even a mosque, but truthfully this visit would be primarily motivated by a desire to see the cathedral.) Thoughts?
Worshippers are always welcome at most any church, temple or mosque. However, you're question is how much can you see of the cathedral once the services are over. I've been there but not during services so I'm not sure. CLICK HERE to see the St. Paul website. Perhaps you can find the answer to your question there. Be sure not to miss the whispering features of the upper dome and the lower levels where many famous people are buried. St. Pauls has one of the highest domes in the world.
Molly: You said Sunday is the only day you can go. But St. Paul's is closed Sundays except for worship. If you can only go on Sunday, you'll be limited to what you can see from wherever you're sitting during the worship service. That's too bad, because there's a lot more you won't be able to see. I'd go on any other day. You can see it in an hour, although allow 2 hours if you want to attempt climbing the dome, no elevator and it's 500+ steps! (This is just a footnote but I thought might be of interest to you: I couldn't be sure but it looked as if your original post was assuming that St. Paul's is Catholic--it's not, it's England's national church and that makes it Church of England/Anglican.)
We went to an evening service at St. Paul's last year, and once the service began the seating area was "closed" and no one else was allowed into the roped off seating area. I did look behind me to see that many people had congregated at the back of the cathedral during the service outside of the roped-off area where we were sitting.
It was difficult to actually hear the words that the pastor speaking because they have a PA system and whenever he spoke his words continued to "ring" throughout the cathedral for a few seconds. But the postlude was a big Bach organ piece which was very nice. Some people left during the postlude, but some continued to sit and listen (me) until it was finished. There were some people sitting and TALKING during the postlude which was QUITE annoying not to mention rude!
Evensong is wonderful at St. Paul's - if you get there early, you may get to sit up in choir, across from the boys' choir; don't know times/days.