There are 4 main networks in the UK. o2, EE, Vodafone, and Three. Everyone else is an MVNO and are “piggy backing”.
Some are even owned by the networks themselves and offer lower prices to cater to more budget conscious customers. For example, Talkmobile is owned by Vodafone (and uses the Vodafone network).
However you can find deals not available on the providers website by using tools such as Martin Lewis’ Money Saving Expert SIM finder.
For an unlimited data sim, Talkmobile sells it 12mo contract only for ÂŁ32.