I know of someone who came to the uk under false pretend to get permanent residence and a british passport,which they now have.

Report the person to your local authorities. They should be able to move it up the chain to the proper people.

The local authorities will tell you to report it to Immigration - https://www.gov.uk/government/organisations/hm-passport-office

What is your proof?

The local authorities will tell you to report it to Immigration - https://www.gov.uk/government/organisations/hm-passport-office

What is your proof?

The fact that the man broke the law, and you usually report that kind of stuff to your local authorities. If they tell him to call immigration then they should be able to give him information on how to contact them. So it was still a good suggestion.

Rather than move it up the chain, I'd start at the top of the chain - at any rate, that's the site and my research appears to indicate the "locals," same as in the US, have little/no interest because no local law was broken.

I only have research, no experience in the matter so your experience very well trumps my research.

And it's all going to boil down to PROOF which is very, very hard to come by in these situations. These reports are very often "sour grapes."