The guy in the video explains exactly that.

If you are a UK citizen, they will charge you under the terrorism act if you refuse to answer their questions or if you try to conceal anything or if you refuse to provide all the passwords to all your devices.

As for what happens when you can genuinely not remember your passwords, I honestly don't know. The guy in the video just gave them access to everything because he travels a lot and did not want to have to explain why he has a terrorism charge to his name every-time he has to apply for a new visa.

Maybe you can refuse and stay silent and fight the charge later in court with a good lawyer but who knows. IANAL so do your own research before doing anything.

As for what happens if you are a foreigner and refuse to unlock your devices/ answer their questions, they can deport you and potentially ban you from entering the country for a certain amount of time or indefinitely.

> Maybe you can refuse and stay silent and fight the charge later in court with a good lawyer but who knows. IANAL...

You ask who knows. Then you say IANAL. I'm excited for you. You're so close to identifying who might know! Keep doing your own research, you got this.