Hello Chris, I'm a big admirer of the Bracknell Half - it's a top event, all credit to you and your team.
I have several of the T-shirts, including one from 2003, all of which I wear to this day because they're good quality. Other race T's have fallen apart.
I'd suggest something that people will use or wear, not just put on the shelf and forget. I've got a Bracknell Half water bottle somewhere and a couple of gym bags (none of which I use, but that's just me).
How about a buff (or generic version thereof, like the Oxford Comfy)? But I'd still be happy with a T-shirt. A hand towel - maybe.
You might want to ask the question on the General forum btw, it won't drop down the page so quickly.