I says asdas because it;s there shop I'm going to, like if I was going to Mr Smiths butchers I would say I am going to Mr Smiths not I am going to Mr Smith, we say Boots because Mr Boot ownes it, we say Marks and Spencers because Mr Spencer was Mr Mark's partner, we say Harrods because Mr Harrod started it.
Anything we don't add a personal 's' to is rare, I say I am going to the Chemist but in my daughters village they call it Lloyds but that's because it is Lloyds chemist.
I used to work for Howellls Dingles and Jollys stores BYW.