If someone in Britain offers to rent you a homely room, you shouldn't be offended. Here's why.
Kits wrote, "I watch some British shows, mainly home purchase or decor, and I often hear the word 'homely.' To me, 'homely' means ugly or at least unattractive, and I think the Brits must mean what I would call 'homey,' meaning cozy, comfortable, etc. Can you explain how 'homely' came to mean the total opposite over here and how 'homey' arose (and why it isn't used over there)?"
It seems that homely underwent a progression in meaning that survived in the U.S. but didn't survive in Britain.
In Middle English, homely first simply referred to something of the home; later it came to mean a simple or ordinary home. Then homely could also mean a simple or ordinary person—perhaps someone who was good around the house at ordinary domestic tasks or was unadorned, plain, or unsophisticated. Finally, all those words—plain, simple, unadorned, unattractive, and by extension, ugly—were applied to appearance and you could describe a house or person as homely, and it was a bad thing. That “unattractive” meaning was used in both Britain and North America for a while, but today it mainly survives only in North America.
I can't explain why the "unattractive" meaning didn't stick in Britain, but it's good to know that if you’re an American traveling in Britain, you shouldn’t be offended if someone offers to rent you a homely room.
An interesting etymological note is that in some cases, homely was closely tied to humble, and long ago it was sometimes spelled with a u: humly.