The listed packages doesn't really say, as they are not 100% complete.
The Drive Package has ACC for 2019, but don't recall if it had to be the Driver Assist Package for 2017.

Do you have access to the D5?

If so look at the front windshield and steering wheel.
The steering wheel should have the controls on the right side to increase or decrease the following distance, see
Land Rover Owner Information

The windshield will have 3 cameras (IIRC) facing forward.

 

I have a 17 HSE. Below are from my window sticker. I believe there was a Drive Pro package that has the adapt cruise control.
Vision Assist Package
LED Headlights w/LED Signature, Auto High Beam Assist
Auto-Dimming Exterior Mirrors, Surround Camera
Autonomous Emergency Braking

Cold Climate Package
Heated Washer Jets, Heated Steering Wheel
Heated Front and Rear Seats

Seven Seat Package
7 Seats, Twin-Speed Transfer Case, Electronic Air Suspension

Drive Package
Blind Spot Monitor and Reverse Traffic Detection
Traffic Sign Recognition and Intelligent Speed Limiter
Driver Condition Monitor

 

The adaptive cruise control is only offered as part of the Drive Pro package on 2017 HSE and HSE Luxury models per my 2017 Discovery sales brochure.

 

Same on the 2018
And I would strongly recommend drive pro. It's very good