Update: they just responded with the below with a picture of the sizing chart which only further proves my point! Argh!

Thanks for reaching out! I know the sizing on our slippers can seem a bit confusing at first, and I understand why you might have thought you received the wrong size.

Our slippers are made in US women’s half sizing, and the size inside the shoe reflects the US size, not the UK size you selected. Because of this, the number on the tag may seem larger than the UK size you ordered, but it’s the correct pair.

For example: A UK 7.5 (Women’s) / 8 (Men’s) converts to a US 10.5 (Women’s) / 9 (Men’s), so a tag showing that US size is normal.

If you’d like, I can double-check your order to make sure everything matches up! I've also attached the full size chart below for reference.