Yes Boxtops, we're all so used to booking online or by phone that we've forgotten there's still a traditional Box Office open from 10am until curtain up each day. Local people and visitors to London book this way.
If I book online I ALWAYs google the theatre I'm going to and find their own website. Sometimes I'm redirected to Ticketmaster or seetickets once i start to book ticktes, but occaisonally they have their own booking process and the closer your are to the theatre itself, the lower the fees.