User request: add ability to contact PM

[MyfanwyNixon]

Your excellent who to write to list seems to be missing an option to write to the Prime Minister.

The PM has a unit dedicated to responding to letters addressed to her at No.10 and the No.10 website has an email-the-pm web-form soliciting correspondence - https://email.number10.gov.uk/

I very much encourage you to add the PM as a Who? option.