I still cant agree with that mate, just because they websites tend to contain assurances it don't mean a thing in reality, I work for a law firm and the amount of cases you would see against those big boys for breaking those assurances or trying to find loop holes is unreal mate.
The smaller outlets I would trust not all but most, especially if you have known them for a long time, one of my mate is a butcher and he does admits there are short cuts for both big players and the small outlets but lot of the small outlets get decent quality meat, especially when it comes to Halal I am very picky and vast majority probably are as well.