I can start, and Sophie can finish.
The proportionality is that you're looking at the action that government is taking, which is the taking of the passport, and weighing it against what injury it might give, what harm to the individual, and then ultimately balancing those against any potential outcome. If the individual is going to engage in a terrorism offence abroad and you know that taking the passport will prevent harm abroad, that's part of the test that either a judge or the minister would have to consider.