It's an offer, no one is forced to order anything - it's not like it's an entrance ticket.
I know part of your point is defeated if you think you need to defend it, but I will take that risk - don't think Teemill is overpriced, if you think away the time invested to create the design and post it online for people to see. They're basically offering something like screenprinting but not with the one-time preparation fees usually applicable. They have a high-tech printing technology and the clothing isn't just your cheapest Bangladeshi product either.
Teemill arranges printing in a super-fast manner, regardless if you order one or one hundred t-shirts, and post it worldwide. If you tried your hand in arranging things for others to enjoy - in this case at no benefit to yourself other than the feeling that you've done something for others - you would appreciate the fact that you're paying money for someone's time and effort.
I'm not promoting the product or asking anyone to change their mind. I just want to explain the thinking from my point of view.
As a side note, it's all done in the UK and all the money stays in the UK, so you're essentially buying British if that still means anything else than a bumper sticker.