You can write to Dan at the following address below, this has been updated as of March 27, 2016
4A Exmoor Street
London W10 6BD
ARG Website: Artists Rights Group LTD
How to send your fan mail and autograph requests to Daniel Radcliffe?:
If you want to request an autograph, follow the guidelines below. If you just want to mail a letter with the address above, and do not want anything back, then you can stop reading! If you live in UK send a properly stamped and self addressed envelope (minimum size 8.5″ x 4″) with your request letter and a photo. You can include a piece of cardboard to keep the photo from bending in shipping and also add “Do Not Bend” on the envelopes. Send your letter and wait. On average, there is going to be a 3+ month wait for a response. If you do not live in UK, you can purchase your British stamps at your local post office.* If you are successful you should get a pre-printed autograph photo from ARG Talent.
Read more here: fanmail.biz