Sorry, out of luck. DL did not really increase the fares on this route, the cheaper fares just have an advance purchase requirement (as noted above). The Z fares (ZHWB68US) you were looking at have a 28-day advance purchase requirement and you just passed that window for the July 17th departure. Same goes for the Premium Economy G fares (AHXB78US). The only way to get these now is to book a departure for July 18th or later. Always pay attention to the fare rules (which are linked on the checkout page).

There's a $2022 Premium Economy G fare (21 day advance purchase requirement) for the dates in question. It seems you are only considering the 8:15AM departure daytime JFK-LHR flight (which doesn't seem to have any G bucket inventory, so it is pricing as a P fare).