I'm not saying that signing is any better than chip and pin, but at least if someone finds my card they will have to attempt my signature and could only use it in shops.
If some one lifted my purse out of my bag after watching me input my pin number in a shop then they could empty my account or use my card in store....
The self service checkouts in Tesco use it, as do the self service machines in all the Odeon cinemas I've been to, not to mention the self service machines at my local Argos Extra.
I guess they have some sort of CCTV on it so if you dispute the transaction later they can verify it for you...sorta like ATM machines
All stores have a fraud limit. Up to that point they will not question things like authorisation. You mentioned about it cutting off after £20 - that is probabl Asda Petrol's fraud limit.