Articles in this case all say that the illegal worker only provided a photocopy of his alleged British passport.

I.e. the employer did not properly and seriously carry out the checks as he didn't ask for the original, hence the heavy fine.

"The business did not see the original copy of the man’s passport, which its owner, Mark Sullivan, said was a “clerical error”" [1]

As I commented previously it is hard to counterfeit a modern British passport in a way that looks genuine and obviously any checks require sight of the original passport...

[1] https://www.theguardian.com/uk-news/2025/jul/22/surrey-chipp...

I literally just got a job with a big British corporation and all they wanted was a photocopy of my passport, no one checked the original. So this practice seems at least relatively common.