Most people in the UK have a mobile phone and as in the US, there are a number of service providers and plan options. The main mobile providers are:
Shops selling phones and plans can be found on most high streets. Knowing how much you intend to use your phone and what you intend to use it for (calls, emailing, texts, web browsing) will greatly benefit you when you try to decide on a plan. You do not have a legal right to keep your mobile number if you switch carriers in the UK.