Various hypotheses. Money laundering, VAT scamming, tax abuse, or just outrageous profiteering based on never being around when the bill for VAT or business rates come due.
There is one famous, very real, very popular American candy store in London -- Kingdom Of Sweets -- which is absolutely legit and has been trading for twenty years, having started elsewhere. It was famous in British geek culture for being where you bought all the sweets you heard US geeks talking about on Foothills and Surfers and the like.
And then there's a couple of likely legit competitors, and then... a whole layer of fakery, fraud and shell companies.
If you want American candy, sure.
(To American HNers: this isn't a joke about you. Or even really that much of a joke, alas)