British Airways runs one non-stop flight per day from London into Denver International Airport, taking 10hrs. At the time of writing the fare was about £750 return.
The drive from Denver to Estes Park takes about 90mins. If your final family holiday destination is one of the resorts off I-70 (a longer haul), we advise booking a hotel near the airport or in central Denver for the remainder of the day, or maybe even two days!
If you opt to hire a car (strongly recommended), consider transitioning out of jet-lag in Boulder, an all-American city that is home to the University of Colorado and recreational opportunities galore (have lunch at The Sink where, according to local legend, undergraduate Robert Redford swept floors). Or take the 1hr flight on United Express from Denver to Eagle County Airport, 50km from the ski resort of Vail.