There are a number of mobile providers in the UK. Some of them actually supply their own networks, with cellphone masts and all the network infrastructure to support that network. The other providers are called Mobile Virtual Network Operators (MVNO) and these providers really just provide mobile services on the main networks and do not supply their own networks.
The main network providers are Vodafone, O2, EE and Three. These providers all supply their own networks which includes masts and network infrastructure, as well as mobile phones and contracts.
(The EE network is a merger between the T-Mobile and the Orange, so if you have one of these old SIMs, they now work on the EE network.)
The other providers (MVNO) simply sell phones and plans but they do not provide mobile networks. They simply piggy-back on the main networks listed above.
So below I have listed some of the main MVNO providers and what networks they are on.
|O2, Vodafone, EE
There are obviously quite a few MVNO in the UK and I have not listed them all, but these are the main providers.
The last entry on the list is interesting in that they use multiple networks - might be good if you have dead spots at home for instance.