The are four networks here in the UK:
- EE (Everything Everywhere, a partnership between T-Mobile and Orange)
- Three (Originally a 3G-only network but now offering 4G)
- O2 (2G, 3G, 4G)
- Vodafone (2G, 3G, 4G)
With the exception of Three, all networks host a range of virtual networks, which are too numerous to mention. There are some that offer great UK rates, and others offer great international or roaming rates.
As far as PAYG is concerned, Three offers unbeatable rates on its 3-2-1 tariff. 3p/min UK calls (to landlines and mobiles), 2p texts, and 1p/Mb data. A little advertised fact is that if you use special access codes, Three offers some of the cheapest international calling rates, too, great for keeping in touch with home.
If you tell us your needs, we can recommend a suitable network for you.
I'm including a link to a pdf document detailing the
3-2-1 tariff. Page 21 explains the international access codes (3p/min to USA, 1p/min to Canada, for instance)