Why Do Americans Say ‘Uh’ and ‘Um’ and British People Say ‘Er’ and ‘Erm’?

I’ve been reading the Harry Potter books with my daughter recently, and while most of the British terms in the series (git, nutter, prat, puckish, peaky, mental, chuffed, having a go, and so on) roll by without making me skip a beat, I keep getting hung up on those little pause words, er and erm. It feels wrong to say them that way, even in my dodgy approximation of a British voice.

The reason it feels wrong to say them that way, is because it is wrong to say them that way. British people do not read er and erm in the way that Americans would read those words, with a fully articulated r. Most British dialects are non-rhotic; the r is not pronounced in words like her or term. So how would a British person pronounce er and erm? Basically, as "uh" and "um," with perhaps a bit more tension in the vowel.

The identity of er and erm depends on pronouncing them like a native in your head, something we don’t have to do with other words (chuffed reads as British even if our internal pronunciation is American, and we can accept that Harry Potter will be pronounced differently by an English person even if we don’t actually pronounce it that way internally when we read). This fact, which on the surface seems so obvious, can escape even the most astute readers. Lynne Murphy of the blog Separated by a Common Language is a linguist who had been living in the UK for years when she finally realized the truth about er and erm. She was watching TV and noticed that the captions for American shows with American actors speaking with American accents used er and erm for uh and um. They were different ways of writing the same sounds. I completely identified with her reaction to this discovery: “Before any of you complain that I should not have been allowed to have a doctorate in Linguistics if it took me this long to figure out something this basic, let me tell you: I've thought the same thing myself. I think the technical term for this is: Duh!”

Or should that be der? In fact, according to the OED der has been a British variant for duh since 1979.  Not sure if it shows up in the Harry Potter books this way, but I’ll be sure to notice it now if it does.