I am pretty sure that when BBC UK say "massive presence" , they are more than likely talking LOTS and LOTS of merchandise in the BBC America booth.....
Im sure (as posted earlier) Matt, Jenna, the showrunners Moffat and Skinner will all be there .. YAY!! (Still to be confirmed

) ..
However, for anyone else representing the show, that is additional costs to BBC America/UK, which is not financially viable for them to do..
However (again!!

).. We can hope for a lot of "private" stalls/booths to pay to bring out any actors ... they would charge a fee for photos/autographs to recoup costs..
Hopefully Im wrong and there is lots of actors etc linked to Doctor Who coming out to Comiccon in July... and if they do, tap the tall weird-looking Aussie guy in line and say "You were wrong" .. 'cause I would be just as happy as everyone else.

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