According to the ELSPA, "The supply of these items is an infringement and an offence under the Copyright, Designs and Patents Act 1988 and the Trade Marks Act 1994." So, how did they get onto shelves in the first place? Clearly the yawn-inducing law behind it all is more complex than simply "they are illegal," otherwise the shops wouldn't still be selling them. Which they are.
Long story short, it's a mess of regulations and red tape, and we've heard it all before. It's hardly a crystal-clear situation where legal teams "will take action where and when appropriate." If a blanket ban of selling flash carts suddenly falls over England, and police start breaking down doors of R4 traders, we'll let you know. Until then, get out there and buy some good games, and use any external device responsibly.