Originally Posted by Jzlerner

Pretty much correct. I'm not going to delve into the exact laws but three things to point out:

1) fruit and veg is kosher HOWEVER must be prepared with a clean uncontaminated knife.
2) most drinks are kosher, at least the mainstream cokes and Pepsi's and everything those giants make.
3) lard. Lard from a pig is not kosher, however, kosher versions of lard from kosher animals eg a cow are available.

I wouldn't be able to say whether it's cost effective for airlines. On the one hand it definitely costs them more than regular meals. How much more? I don't think anyone can accurately price that as it depends on too many factors. However, if BA stopped offering kosher I would without a doubt switch to another airline.