There are two entirely separate issues here.

The first one is that of animal welfare. In the UK the law requires pre-stunning, and ritual slaughter on religious grounds is only permitted as an exception. Most people assume this is because pre-stunning is considered to be more humane - whether or not this is actually the case (and the debate seems unresolved on this), this is the widespread perception, reinforced by the views of organisations such as the RSPCA.

If I visit an Asian restaurant then I might expect it to serve halal meat, but that is something I can choose to take into consideration. What is upsetting people is the revelation that meat slaughtered in a way which they consider to be less humane is being served without their knowledge and in situations where they wouldn't necessarily expect it, simply because those organisations cannot be bothered to go to the effort of providing separate menus for Muslims.

I choose to eat meat. I would prefer that the animals have been reared in decent conditions and slaughtered in a humane way. If I go to an Asian restaurant I have to consider the possibility (even if it isn't announced) that they may use halal meat but I can decide for myself. If non-Muslim organisations are also serving it in circumstances where I wouldn't expect to find it, I cannot make that choice.

That is quite different from the other, less acceptable point of view, (which I suspect underlies the Daily Mail's message even if it isn't explicit) which is that this is another example of the feelings of the Muslim minority taking priority over those of the rest of us. It is deplorable that an issue of animal welfare should be used to whip up anti-Muslim feeling.