Not St. Paul's
Over the summer break, I ventured out into the city centre and joined in with a couple of meetup groups sketching London's architecture. The first visit was to St. Paul's Cathedral, and the second was to the Barbican. Overwhelmed by the cathedral's size and variety of features, I just gave up and drew a different building in the vicinity of St. Paul's instead.
And, yes, I did use a ruler. I like straight lines.