> So I was born in the US, where I learned "glacier" was pronounced "GLAY-sher".
> Then I moved to the UK and learned that it was pronounced "GLASS-ee-er".
> Now apparently the UK pronunciation is "GLAY-see-er"? When did that happen and how did I miss it?
Hm… Rhotic pronunciation on the islands? I bet it is a slip of the keyboard or you are somewhere in between Somerset and Devon. And it looks like you do not travel a lot indeed. Yes, you need to drive several thousand miles in the US to encounter a frying pan, a skillet and even probably a spider. But you usually need to walk just a couple of blocks to hear glAHseeə-glAseeə-glAYseeə-glAsee-glAsyə-etc. in the UK. And if you decide to drive you shall hear more variants even with zh in between.