I believe this flight is a continuation of LHR-LAX. The flight # "UA 935" is used for all segments LHR-LAX-LAS so this flight is sold as 3 class while operated on a 2 class aircraft. Passengers booked in business class are simply seated in the first class section of the plane so you'll get the standard domestic first class.

Have you ever heard of the AA flights that are sold as 2 class but operated with 3 class aircraft? Well, this is the opposite of that.