In many parts of Europe it is compulsory (on paper) to carry identification. However, this often dates back to Nazi occupied times (e.g. France) and isn’t enforced anymore. When it is enforced it is quite often by corrupt cops (esp in parts of eastern europe) looking for a chance to ‘fine’ you.
I used to live in France and ID cards are a bureaucratic nightmare… One French friend went to the town hall to renew her 10 year ID after it expired. It took her 2 years, many wasted days and many headaches to ‘prove’ that she was French. I can’t begin to imagine how much harder it was for non-EU immigrants.
I did have a french work permit (carte de sejour) but never actually needed it. The one time I did try to claim some money back from the state I didn’t need to show it. I had to show my parents birth certificates to get one. I showed them a random bit of forged paper instead.
the UK version is either worrying, or a complete waste of time, or both.