un1corn,
I believe a more likely scenario is that Asda could see local shops selling halal meat flourishing. If they then switch over to supplying halal too, not only would they increase meat sales (assuming that they've factored in the drop in from non-Muslims to the increase of Muslim customers), but they would ALSO see a rise in sales of other products.
The thinking would be, "Well, seeing as I'm here buying lamb for dinner, I may as well do all the rest of my shopping".
In an area with a high Muslim population, that would be a serious 'quids in' for Asda.
Finally, I have to take issue with your Wall's sausages stance. Sainsbury's 'Taste The Difference' pork Cumberland sausages are MILES nicer:
http://www.mysupermar..._6_per_pack_400g.html
Also, the Sainsbury's bangers are 83% pork meat, as opposed to Wall's 64%.
The defence rests m'lud...