It's all very irritating- if we're going to have different words for "<" that apply to countable items and measurable quantities (thus showing that English speakers have an innate understanding of number theory), then we ought to have two words for "more" as well. I'm going to check with DD when she gets back from Germany whether they try the same daft trick; the French don't, and I think it's because they know they'd be barking up the wrong linguistic tree.
Yes, pounds can be counted, as can pennies, days, months etc. But that's not to say that there isn't a difference between a number of items and a quantity of something; money, time etc. So if you're asking "How many pound coins do I need to operate this machine?" the answer could be "Fewer than 10", whereas if my DD, with a tenner in her pocket, asks that well-known rhetorical question "How much will I need to buy lunch?" the correct (grammatically as well as parentally) answer is "Less than ten pounds". Just as it would be if she had £10.50 in her pocket, when it would be "Less than £10.50", except in that case it's more obvious that only the word "less" can be used, and you don't get sucked into faux-pedantry by the juicy option of saying "Fewer than £10".