I've implemented point-of-sale systems before, and lemme tell you, there's nothing magical about a magstripe reader, a microcontroller, and a bit of firmware.
Think for a second about recent credit-card purchases you've made. If your habits are anything like mine, aside from gas stations (where the reader is usually integrated into the pump), most card "swipes" happen through either a peripheral device connected to a generic PC, or a compact card reader with little more than a keypad and phone jack, with no visible branding, certification, or tamper-resistant seals in sight.
(Also most places in the UK will not swipe your card anymore. It's all chip+pin).
And if they roll out person-to-person, this would be a whole lot nicer than, say, having your friend write you a check.
Maybe this is a European thing, but here in London every restaurant has a number of portable card readers. (Everyone here also uses chip & pin, though, too.) As a consumer, it's awesome... much faster than the old days, and a hell of a lot easier to split the bill when you want to do that.
Perhaps the London merchants are required or otherwise incentivized to use portable ones?
I guess I'm wondering what use-case it's aiming at.
Also I absolutely would not trust someone swiping my card through one of these. I like a closed box that is obviously provided by a bank, which cannot be tampered with. Not an iphone that could be doing anything with my details.