Fair play to Tescos for publically admitting it.
Just because the others don't shout about it, doesn't mean that they don't sell it (or buy from suppliers that do) 
Indeed. In fact it's often said that a lot of meat is Halal, but just not labelled as such. There is pressure for labelling to be clear on this.
It niggles that shops will go on about vegan, gluten free, ethical eggs, free range chicken, line caught fish and everything so 'Nice' and ethical and cuddly . . . but then advertise their Halal meat on TV. Actually, the supermarkets claim that their Halal is humane blah blah. I just don't believe them. It's Halal or not Halal.
There are some terrible videos around of Halal slaughter, but it is never stated which country. I'm not some rabid animal activist. I like meat. I just think that in this day and age there is no excuse for brutality and needless suffering.