They're usually cold, damp and dark places, you're sometimes better off in a tent outside
I only know of a couple of in the Lakes and neither has a firewood supply to hand, you'd have to think about taking a bag of coal or logs with you if you want the place warm.
The Scottish bothies I've used are usually clean with good sleeping platforms, set in some stunning locations too.
Glas-allt-Shiel, queens bothy, us commoners us the entrance round the back
Gelder Shiel
Ruigh-aiteachain, lovely setting
All the mod cons ..
Melgarve