@anon5352 (JR Musselman) - You should contact the Metropolitan Police Station that is the closest to the address of the person in question.
There is loads of information by the Fraud Squad on their website that should be very useful to you. It’s very easy to use and covers a wide range of fraudulent crimes and how to handle them.
If you’ve been the victim of a crime in excess of 75,000 pounds or a 419 fraud then you should call the local switchboard and ask to be put through to the Fraud Squad.
If the crime is less than 75,000 pounds then you can go directly to the nearest London Metropolitan Police Service station that corresponds to the phone number you received.