This is a solid opportunity for families planning a spring trip to London. At 40,000 miles per person, you're looking at a meaningful discount compared to the roughly 60,000 miles United typically charges for economy awards on this transatlantic route. The low $6 tax figure is also notable—London flights often come with much higher carrier surcharges depending on the operating airline.
Availability appears to be wide open for travel on April 20, 2026, which gives you plenty of time to coordinate schedules and accumulate any additional miles you might need. April is a pleasant time to visit London, with milder weather and fewer crowds than the summer peak season.
One practical tip: if you're booking for a family, search for your full party size at once rather than booking one seat at a time. Award availability can disappear quickly once you complete a booking, and you don't want to end up with half your group confirmed and the rest stranded at a higher rate.
How to book
Search and book directly through United's award search tool at https://www.united.com/en/us/book-flight/united-awards/search