Block-the-blocker only blocks "number withheld". International calls are "number unavailable" and they get through without a problem - your phone may display them the same way, but the phone network sees them differently.
"Unlisting" isn't relevant - the spammers aren't looking people up in phone books to call them - they are just dialling all possible phone numbers.
The reason you don't get spam calls in the UK is primarily because phone calls aren't free, even for local calls - again it's nothing to do with being ex-directory or not.