A restriction to "members-only merchandise" is wrong, doubly so with current low membership levels. When you're talking about a 10k+ member base, then I can see a point to members-exclusive merchandise; this complementing a range of items available to the wider public.

I agree. The only thing is to add really is that branded T-Shirts might be good for WMUK people to wear at (informal) events where they represent the friendly face of Wikipedia. But the market is far, far too small at this moment in time. By the way, are there WMUK T-Shirts on Cafepress? Better to let them handle things at the moment, even at a premium.