Not sure about the plaice, but in a portion of chip shop chips there are 9 UK WW points.
Not sure about the WW centres as I live in the wrong end of the country.
Most of the big supermarkets, Tesco, Sainsburys, Safeways, Asda, Co-op, etc. stock some of the WW brand food. Personally I think Asda's is the best as they stock more of the goods than anybody else.
Hope that is some help - will keep my eyes open for the plaice points