If you were blackmailed to write a good reference for someone who is now a Police officer because of your reference, could you be fined or imprisoned if you confess this to his workplace?

I was covertly blackmailed to writing a reference for a someone who was a childhood friend and was holding a lot of my money that I had lent him in the past. I need genuine legal advice on what will happen if I confess this to his workplace as this is causing me anxiety.

This happened in the UK, London. And this reference was for him to get a job in the Metropolitan Police which he ended up getting. I need your advice on what could happen to me if I do confess this to them?