Getting cheap flights

A world cruise is delightful, only unpack once, but we still need to get to the start of the cruise (San Francisco), and back from the end of the cruise (Southampton, England). With applications on the web like Expedia, one would think that comparison shopping for airline tickets would be easy. However, in looking for a cheap flight to San Francisco, Expedia missed half-cost fares offered by Virgin American. (A week later it found them, although I don't understand the time delay). Likewise, British Air one-way tickets were quite expensive, but a round trip (even if you don't use both legs) was cheaper than one-way. Perhaps it's time to rely on a good travel agent.