Asda Mobile is definitely worth a look. (I've recently bought a new phone although I've not even taken it out of the box yet. I'm definitely going with Asda PAYG though).
You can either buy 30 day inclusive bundles
https://mobile.asda.com/bundles
or just put some credit on your phone and then pay 8p per minute for calls, 4p for texts and 5p per Mb of data.
Unless you'll only be using the phone very occasionally (and hardly ever for web browsing, etc), bundles are probably the best option. There's a handy calculator here to help you decide which might be best for you:
https://mobile.asda.com/bundles/calculator