It's not just about price but also about service. If you go to the main mobile operators such as EE, Three then thats the service you get. If you go with the cheaper Mobile Virtuale Network Operators such as Tesco, Asda, Smarty,etc then you need to check what provider they are using to ensure you get best coverage.
Equally, you have to be aware that MVNO's that may sit on the back of EE for example can at anytime change their provider. For a good while we used a PAYG from Asda because it was cheap but also used EE which is one of only two providers we can get a signal from at home - sometimes. Then Asda decided to switch to Vodafone as provider and we could not get any signal from them, so we switched to someone else.
My phone can accomadate two sims so I can use EE and Three which are the only two services we get at home and in most places we visit.