The old saying is proving itself true. March comes in like a lion and out with a lot of flight deals. Impressive deals have been frequently popping up, like $171 round-trip tickets to Hawaii, which were, of course, sold out almost instantly. (Though, you can find $300 round-trip flights in a new sale.) This deal isn't quite that jaw-dropping, but an STA sale will allow you to fly round-trip to London for less than $500.
STA Travel, "the world's largest student and youth travel agent," has partnered with Visit Britain to offer 18 to 35-year-old travelers bargain-priced tickets to London through March 31. (There are non-age-restricted deals below this one, as well.) One-way flights can be found for as little as $163. However, return flights aren't quite that cheap.
Nonetheless, you'll find round-trip tickets from Boston for $426, New York for $443, Chicago for $452, Houston for $460, San Francisco for $473, and Los Angeles for $594. The majority of the discounted flights are through airlines that won't ding you with other fees. Airlines on the low-priced tickets include American Airlines, KLM, United Airlines, Virgin Atlantic, and Air France.