Here in the UK all networks (Vodafone, O2, "3", EE) use similar frequencies and are all GSM based. All phones bought outright from any where are unlocked and can be used with any SIM from any of the networks (Tesco use O2, Virgin use Orange, EE or T - mobile, EE are Orange and T - mobile's LTE). If you go get yourself a contract the phone is unlocked (didn't used to be in the past) and usually "free" if the contract is purchased direct from a network the phone usually runs carrier branded firmware, which for all intents and purposes is stock with network added bloat ware. A contract from a third party the phone is stock (usually).
The phone is legally not yours until the contract is up, but keep paying the monthly bill and you can do what you like with it in reality.
Contracts are over 12, 18 or 24 months a typical one for say a Samsung Galaxy S4 would cost around