In UK foreign students and tourists, two main international telephone networks can be advised: Lycamobile and Lebara. These networks provide the cheapest calls around the world.

Where and how to buy a sim card?
To buy a SIM card, you do not need any passport data, no additional ID cards. We went to the store, paid for a sim card with a certain balance, put the SIM card in the phone, call wherever we want.
Buy sim-cards you can in the main network of retail stores. Below is a basic list of these stores.
Also you can buy SIM cards in the so-called "corner shop" - they are small shops selling basic food and household products. Either ask about these sim cards in the airport hall or hotel.