Maybe I did not make it clear. I live in Spain where carrying an ID is compulsory and the only acceptable ID the Brits have is their passport, so they / we must carry it. That applies to people living in Spain and visitors but as I said many people 'break the law' and carry a copy or an alternative document. In most circumstances we get by with that.
We used to all have a Residencia card but unfortunately they changed the law. An ID card is so easy to carry so convenient to have. Not having one is a problem.
The ID card the Spanish have will allow entry to various countries including UK and Ireland but the point I was making was if an ID costs around a fiver why should a UK one be so expensive ?