Do they have contacts on their IMdb?
I dunno.

If I were you, I'd try the agent addresses Sherlockology recorded down:
http://www.sherlockology.com/characters   Just have to click on the link of their 'real' name for RL bio/info.  Tried it once.. was disappointed no reply from some, but Rupert sent me back a signed photo!!  (using, of course, the already self-addressed and stamped envelope I included for).

A belated 'hey'!  Yeah, I know, it was really nice/cool!  I've always just loved stuff 'from afar' and never thought to write to my favorites (and it's so different now with being able to ping some of them a tweet!  But that's a whole other thing...).

I know getting a lot of fan mail must be interesting, though, and all the varied stuff they're sent!  And sure, telling them how much you love their stuff with the hope of a reply or pic, or just to let them know they're enjoyed!... after hearing more about it from other fans online when I started getting into fandoms more, many years ago I decided I wanted to try it with David Tennant and John Barrowman, my then-crushes from Doctor Who .  Just a short and sweet handwritten letter about being their fan (politely requesting a signed pic at the end), an S.A.S.E., because I knew that made it easier on their busy end (stamped, smaller manila envelope, about 6"x9"), mailed in the same kind of envelope.  And they actually replied!!  With their own promo postcards!  (possibly auto-signed, but I don't care!) 



Soo... some years later when I got utterly lost in Sherlock, I thought it would be awesome to try with those guys.  So using the same format as before, I mailed Benedict, Martin, Andrew, and Rupert (keeping it to just top favorites for first attempt... I know, I know, I should write 'the girls', too!  They might have an easier time replying!).  Except this time, I figure I'd make it even easier and include a pic ready to sign (just picked my favorite promo picture, printed on decent paper, kept small to fit in the smaller envelope, so wouldn't be bent, as you can see in the pic).  And... only Rupert replied.  Which is awesome, but still... darn. (and this was even before Ben was married/with kid! I can't imagine now!) I even heard Andrew say he's always so touched by number of fan letters and tries to reply, but hey... I know they're busy.  But still.  Eee...  personalized, from Rupert!! 
Good luck trying!

I was not expecting such an elaborate reply, Russell, but that is a cool story. I don't know what the postage would be to send a letter to someone in the UK from here in Canada, but I will consider what you have done in the future.