Yeah, I noticed that when I was in London a few months ago. They're almost non-existent in NYC as far as I can tell, and that experience has been consistent with my experience elsewhere in the US.
Perhaps the London merchants are required or otherwise incentivized to use portable ones?
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.