Very few networks offer traditional PAYG SIMs, in which you top up your account and spend the credit as you use your phone. If that's what you're after, your choices of provider are…
Asda Mobile
EE
Giffgaff
O2
Three
Virgin Mobile
Vodafone
Some of them also let you use your credit to buy 30-day bundles of calls, texts, and data - which work out far better value than spending as you use.