As shown in the chart at the bottom of this page the half-life of chlorine at 5% at 75ºF (24ºC) is 1147 days or over 3 years. Essentially, low concentrations of bleach kept at room temperature take a long time to degrade. Here in the U.S. Clorox Regular says 6% as do many off-brand Ultra bleaches.SJSwimmer said:Though I'm away from home, it REALLY seems like all those Clorox containers say something like "<6%" as well, possibly because if it degrades over time that's all they can guarantee?
It is certainly possible that Clorox decided to make only weaker bleaches in the U.K., but they certainly didn't have to do that.
Also, there are quite a few manufacturers of chlorinating liquid at 10% and 12.5%. In Europe, Purex is one such brand of higher concentration chlorinating liquid. In the U.K., Tesco seems to use the vague "< 5%" approach. This link implies that you can buy the more concentrated chlorinating liquid at pool stores so you should compare that pricing against the far weaker bleach available in other stores.