I got the MJF Diamond pass and the shirt that I got with it was the 'Return to the future' with Marty holding a hoverboard in white silhouette with a blue outline. I didn't see anyone wearing a Fox to the Future shirt that I can remember, so I don't know where those came from. Showmasters said that this shirt would only be available through buying the Diamond Pass, so I don't think you'll have much luck finding one I'm afraid.
Thank you for your reply! I guess i'll have to do without one! But i think i just assumed this "fox" one I saw was the diamond pass one, as the diamond pass wording called it a "1 x Exclusive London Film & Comic Con Back to the Future Fox t-shirt" and plugging the return to the future docu doesn't really cover that (as much as I want one though!)
But thanks for replying.