you gonna apply to the last force area where you resided.
If in London, to Metropolitan
email new.scotland.yard@ met.police.uk
If you no idea of which force you gonna contact, they might be give you some help as well, something like the likely police force, and their website. And they are fast in response, to mine, within 12 hrs, though saying in 5 working days.
Finally you need to mail out the document (with your full name, current address, date of birth), the filled form, and 10 pounds cheque, and wait.