Tesla changes its pricing policy on retrograde software activation all the time. This is current March 2020 UK pricing for a car with AP hardware but no AP activated. You can no longer enable 'Extended AP', your only 2 options are as below.
£2950 to enable the same AP features as what now comes as standard on UK cars - so AP stuff but no Navigate on AP/smart summon/FSD etc.
FSD is another £5900 in additional, which enables FSD features and makes the car eligible for HW3 upgrade. You need to buy AP as well as FSD, making the total cost of FSD activation nearly £9k!!
This pricing policy is deliberate, Tesla want to do as few HW3 retrofits to older cars with HW2 AP as possible. Given the total cost of a 2017 car, very few people are going to pay out nearly £9k to upgrade the AP to HW3.
Tesla did do a 'flash sale' of FSD for non AP HW2.0 cars about 18months ago when they dropped the prices for retrograde FSD activation to £4k, which is when I bit and bought FSD for our 2017 HW2.0 car. Tesla MAY do this again, but there is no guarantee.