Many bike shops won't touch Halfords vouchers as the commission charges are so much higher than all the other schemes such as the NHS. Trust my company to have the worst one going. Still it's better than nothing. Although I believe it should just be one single scheme in the U.K.
They do accept it in Halfords though, and they apparently do pretty decent bikes, or so I'm told. Though I'm also told they are sometimes built by pretty poor technicians. Still obviously restrictive, though. And you're right. They should stick to one scheme that any bike retailer can then opt in to.