It really is just a business decision made by a corporation in order to address customer demand = higher sales and profits.
According to one of the linked articles in the OP:
Halal-only menu is in response to 'strong demand' from Muslim customers
And according to the same article, we're talking about 185 of 1500 outlets in the UK - roughly 10%.
To me, it would be over-stepping its bounds if a government told a corporation that it could
not do this. I believe the free market should decide in this case... and in most cases. If people don't like Subway's decision, they're free not to eat there. At least in the U.S. (probably elsewhere too), it's against the law to represent something as Kosher if it is in fact not. And more than a few chains in the U.S. do have Kosher items on their menus. Whether Halal or Kosher, I don't see a difference. :dunno:
This is EXACTLY it.
From a pure marketing standpoint, this just makes business sense. If you've been to any McDonalds, Burger King, or Kentucky Fried Chicken (even Pizza Hut) anywhere else in the world you'll notice menu items from your area may not be on the menu and you'll end up having some pretty weird shit in its place.
For example, here in the Netherlads, there is a McKroket and a Satay burger. Both are gross as fuck, but both the kroket and satay are a dutch junk food. It's part of their culture. Nothing to say it's right or wrong, but it wouldn't be dutch without it. In germany I've actually seen a Schnitzel burger. Not as weird to me since it's like chicken fried steak sandwiches I made as a kid out of left overs. But, it's German. Nothing wrong about it.
Hell, even in my grocery store in my neighborhood we have a lot of Halal items because guess what? We have a huge Muslim community here. In more of the center there's a bigger indonesian community and you notice a lot more foods geared towards them. It's just business. Nothing to be offended about. Oh, and on passover, our local grocery store reduces wheat products and heavily promotes things like matza crackers and other kosher food. Again, we apparently have enough in our community to quantify this as a business decision. Nothing to get pissed over.
Oh, and as for halal butchers....I don't know about the UK or the rest of the world, but halal butchers in the Netherlands often have better quality lamb and at better prices than dutch butchers. Oh, and they'll cut it however I want them to instead of giving me the blank face and "sorry, we can't do it" that I get from dutch butchers. At least they get the whole customer service thing, even from an obvious non-muslim.
In the end, when your customer base starts to change, you need to offer what they want or risk going out of business. In this case, it just happens to be to a highly controversial group of people. If it was because of a large group of gluten intolerant people, nobody would say a word.