I just found the most interesting piece of information on the Customs section of visitlondon.com. Apparently, if you are traveling into the U.K. from countries outside the E.U., you are allowed to bring in 250 ml of toilet water.
Mind you, it must travel with you, and you must not intend to sell it.
They specify six categories on the Web site, including cigarettes, cigars, liqueurs and wine. Of all of the things they could place mandates on and choose to detail in those six bullet points, they choose toilet water? Really? I really don't see there being that big of a market for toilet water. To each his own, I suppose, but that is one rule I don't think I'm going to have to worry about.