> you don't have the right to refuse

What happens if I still do? Or pretend that I forgot my pin and accidentally lock my phone down?

They serve you a RIPA notice and you either provide the password or they prosecute you. Penalties are 2-5 years in jail depending on the nature of the investigation.

I avoid flying anywhere near the UK or its territories.

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.