"British security/police seem to take their power a bit too far very often"...

And that is the actual problem here as well. At the airport you end up in a "your rights end here" zone. According to the original story, the questioned person had no right to a laywer as he was not arrested. But he was also not free to go. So there you are: stuck between a rock and a hard place... Sure, you have every right not to unlock your phone. But you will go nowhere, miss your flight, your connecting flight, etc. And all the officers just go home when their shift is over and forget your case ever happened...