It's almost certainly chlorine or chloramine in your water. I suppose it could be the SO-4, I've never heard of problems with that yeast. That said, there are better British yeasts, IMO.
+1 to Dave's response.
There are a few ways to remove chlorine and chloramine from your tap water. The best way is to use an activated charcoal filter. Adding some campden tablets will also work, and the third way is to let the water sit out for 24 hrs prior which will allow most of the chlorine to come out but that won't allow all of the chloramine to escape. It takes up to a week for all chloramine to evaporate.
Use Campden. That is a very efficient way to remove chlorine and chlorine almost immediately. Filtration and evaporation are both very poor at removing chloramine.