United is touting a Jolly Old London sale on its site, for purchase through the end of business on March 14.
How jolly is it? Supposedly fares, such as Chicago/London for $214 each way before taxes and surcharges, are good until May 17, so our first stab at finding fares was leaving Chicago May 4 and coming back May 13, for which we got a final price of $626 RT. That's not awful, but American and Virgin are charging $510 for nonstop flights on those dates, which is a much better deal.
Checking Baltimore to London ($190 each way before taxes according to UA's site) leaving April 13 and returning a week later, we found $928 RT on United via connecting flights vs. $725 RT nonstop on BA.
Clearly, this isn't much of a sale. We'd skip it if we were you.