Just outside St. Paul's cathedral, you can board one of the great moving icons of London, the number 15, a shiny red, open-backed Routemaster bus, which runs every 15 minutes as part of a heritage service. It is one of only two remaining London bus lines where old-fashioned Routemaster double-decker buses still operate. Route 15 links Tower Hill, Monument and St. Paul's with Trafalgar Square and Regent Street. Although you can't hop on and off at will,unless you've bought a Travelcard, you can enjoy your own self-guided London bus tour for £1.30 per person.