Mobile phone network providers within the UK are ranked as follows:
This was determined in a study of reliability, speed, and availability.
LONG TERM SOLUTIONS
If you do not have an address or a bank account in the UK, you cannot get a phone contract. Check out the short term solutions below if you don’t already have both of these requirements.
A simple mobile phone plan with 2GB of data + unlimited talk and text will cost £24.99 from EE. Beware: this plan is for 24 months and VAT (value added tax) will be applied if you are purchasing a phone.
O2 has by far the best option for calling friends and family abroad. With their International Favourites add on, you get 3000 minutes of landline calls and 100 minutes of mobile calls to 3 international numbers for £10. Or, for £20 a month, you can get 3000 minutes of landline calls and 200 minutes of mobile calls to 5 international numbers.
SHORT TERM SOLUTIONS
You can use a UK SIM card in your current phone if you have an unlocked GSM phone.
If you are visiting for a short time, all you should require is a £20 flip phone with a free SIM card to put money on and make calls in emergencies (£75 will get you a smart phone). About £10 will get you 150 minutes, unlimited UK texts, and 500MB of data. There are even SIM card vending machines in the London Heathrow Airport. I am a big fan of SIM cards because they are the cheapest option if you are just visiting, they are great in emergencies, and easy to use.