"Sir Anthony Hopkins can be contacted through his agents Rick Nicita and Duncan Heath who are based in Los Angeles, USA and London, UK respectively. He can also be contacted through his literary agent Robert Gottlieb, publicist- Paul Bloch and attorney- Craig Emanuel who are all based in the US. Hopkins is an Oscar winning actor with more than 40 years, featuring in movies such as The Silence of the Lambs and Hannibal."
Pick one of these people. They have offices. Send it addressed to him C/O one of the above. Their addresses should be readily available. Go the road less travelled. If you do a fan mail address, it will get lost in the fluff. If you go through their publisher, publicist, agent, or attorney, you have a better chance of them getting the letter. When I sent Gary Busey his, I sent it to his attorney.