Err, they bought a Cheese and Onion Pasty, which turned out to be mislabelled and was in fact a Beef and Potato pasty....I would complain as well. Religion aside, what if the person was a strict vegetarian or allergic to one of the ingredients?
Usual Daily Mail, who have probably blown it all out of proportion and added a slant to make it far more sensational than the truth, which is that the family probably complained, added that as Muslims they could not eat something and then when offered alcohol (which is thoughtless in any case, you simply offer goods of their choice to a value, lots of people do not drink alcohol and it was a child in any case who ate the pasty) they probably told the Manager that was forbidden as well. I expect it was the attitude of the store rather than the specifics that have upset the Khan family, and the DM, as it is its purview has made a mountain out of it to sell copy...and joe public that reads this garbage has taken it all in, hook, line and sinker.
And the argument that commercial Halal is less humane than conventional commercial slaughter has been proven erroneous time and again. People saying that Muslims shouldn't be offended by non-halal, yet getting offended themselves over Halal...madness.
Just let people get on with their lives, whether you are a Muslim or otherwise.