In the UK, the networks are:
- EE (Everything Everywhere)
- Three
- O2
- Vodafone
O2 is being bought by Three, and EE has been bought by BT (BT is the primary landline/broadband provider)
Handsets from Three are all unlocked, but the other networks lock their handsets. Usually you have to pay to have handsets unlocked, but O2 will do it for free if you have a contract phone.
For contracts, EE is probably the chapest; Vodafone is more business oriented, so is one of the more expensive networks. However, if you buy a contract through Carphone Warehouse, contract prices are often much cheaper, and there is the added bonus of all handsets from CPW being unlocked (except the iPhone.) Contracts are all 24 months long.
If you want 4G, I have fond that EE and Three are the best in my experience.
For PAYG, Three offers the best rate the 3-2-1 deal - 3p/min calls to landlines and other networks, 2p SMS, and 1p/Mb data.
There are plenty of SIM-only deals to choose from. 30 day rolling contracts or 12 month contracts.
We are awash with MVNOs. There are too many to mention, but the majority of them run on the EE network, and typically the tariff would be 8p/min calls, 4p/SMS. Of course some providers have slightly cheaper rates, others are slightly more expensive.