United Airlines is offering a “sale” on economy class award tickets to London Heathrow. Flights must originate anywhere in the US or Canada, excluding Hawaii, and travel must be completed by March 31. In addition to the miles, you’ll pay about $75 or $80 in taxes.

This is an excellent deal if you live in smaller cities without nonstop service to the UK. (Fargo to London costs the same as New York to London…) But if you’re flying from New York City, where American is currently offering flights to London for $365 round trip, including all taxes (other airlines are charging just a few bucks more), then you may be better off paying cash and keeping the miles.

However, tickets “purchased” with miles generally have fewer restrictions than the cash tickets. You can circumvent Saturday-night stay requirements, for example.

Bottom line: if you find award availability, be sure to check the cash fare for your specific dates before you finalize any award.

Bonus: An even better limited time offer on United is their reduced economy class mileage award to China, also through March 31. Only 45,000 miles to Beijing, Shanghai, or Hong Kong!