"in absentia and without due process?"
In absentia? Yes. As I have already stated, the ECHR has ruled that Theresa May acted diligently, and in accordance with the law when she revoked an individual's British citizenship and passport whilst he was in Sudan. It has been done on multiple occasions.
Without due process? The due process is having evidence that the person concerned has taken part in, or has planned acts of terrorism against this country, or is an active member of a terrorist organisation which is doing the same thing.
"That is surely not what we are about?"
Protecting our fellow citizens from terrorist atrocities? I sincerely hope that it is very much what we are about, and the ECHR agrees. I'm astonished to discover that apparently you don't.