Tesco has been gradually increasing their halal range for the past few years, many branches have a halal meat section in the refrigeration areas as well as deli products, snacks and sandwiches and wraps. I have tried quite a few of their deli produce like the chorizo, sausages, sliced chicken and pasties as well as the Shazans range of meats which are quite good.
It seems the big supermarket chains are listening to the demand for more halal products.