Autopilot is definitely enough. Currently all that FSD gives you over Autopilot for motorway driving is the ability to change lanes and take exits/intersections automatically.

So if you wanted to overtake a car on the motorway without FSD, you would have to disengage autopilot, overtake and then re-engage. With FSD you just turn on the indicator and it changes lane for you when it's safe.

In the EU (including UK) you have to manually confirm via the indicator that you want the car to change lanes if you have FSD. In the US the car can change lanes by itself without you having to do anything. The EU will hopefully have the same level of autonomy in the future but for now there are more restrictions.