Another vote for Celtic. I got the mule slippers for xmas in grey and am very pleased. Same sole as the pic above, I also need pop out into the garden in them too.
I know you're after slippers, but I've had their outdoor ugg boots too and the quality is far superior to the Ugg brand (can't speak for Emu and all the other brands though)
Also, Celtic are made in GB with British wool.