UK Mobile Phone Networks
To start with there was 4 mobile phone networks in use in the UK. O2 (Bt Cellnet) & Vodafone are the largest and oldest networks in the UK. When they started out they worked using analogue technology which was the 1st generation of mobile telephones. They were soon joined by T-Mobile (One 2 One aka Mercury) and Orange. All of the networks now work using digital technology which is also known as 2nd generation. Digital allows your phone calls to be more secure as the signals from your mobile phone are scrambled so no one could clone your sim card. It also made your reception and call quality alot better. You might be thinking to your self that you have heard of more than four networks, and your not wrong but the other networks excluding Three (we'll move on to them in a second) are known as virtual networks. What these virtual networks do is use other networks technology but have different tariffs and pricing to the network they are borrowing from. For Example Virgin Mobile and easyMobile use T-Mobile as they "buddy network", OneTel use the Vodafone network and Sainsbury's and Tesco use the O2 network. Most of the networks have now launched their 3G services which is the third generation in mobile technology. 3G allows faster data transfer rates and offers additional data services such as Video Calling, Entertainment and location based services. The first network to launch these services were Three they started on the 3G network rather than working their way up the "ladder". Three in a way is half way between being a virtual network and a normal network. As they have their own masts all over the place but due to the way their masts work its taking a little while to get the whole of the UK covered. Therefore when you do not have a Three reception you seamlessly change onto O2s network. But when this happens you cannot use all of the 3G services. All of the networks provide a good coverage, the best of these is said to be O2 & Vodafone. T-Mobile & Orange are sometimes lagging behind due to the bands that their networks operate on. O2 & Vodafone both work on the GSM 900 band, whilst Orange & T-Mobile work on GSM 1800. In the past if you were taking your phone abroad this would be something that you would need to considered but now pretty much every phone you can buy is dual band. The only thing you need to worry about is if you are going to the US as they use a different band all together, known as Triband which operates on the 1900 band. All of the networks have released coverage maps so that you can check the signal of their network for where you intend on using the phone. However sometimes this isn't the most reliable source of information as it sometimes can be wrong. So it might be worth to ask family and friends to see what network works for them in your local area.
Coverage Maps