Bought a pay as you go mobile at Christmas for somebody from the Carphone Warehouse and it was 20 quid. Ten pounds for the phone and ten pounds of credit for the sim card which was on Orange. Pretty cheap and cheerful especially if you are staying short term.
Don't know the best network to have for rural areas of countryside though. The main networks are O2, Vodafone, Orange and T-Mobile so I would imagine it would be one of them that you would need.
I got my o2 sim free from their own website and it's okay I get free same network texts and calls if I top up 15 pounds a month.
Texts to other networks are ten pence and phone calls for the first 3 minutes are more expensive but cheap after that.
Hope that is of some help. I am happy with the deal I get from my phone as I mainly call and text o2 numbers but there may be better deals out there for sims that involve making frequent calls etc.
I think Orange might have a good deal for pay as you go sim cards at the moment.