In reply to TobyA:
For impartial (stated to be) comments try
http://www.moneysavingexpert.com/phones/mobile-phone-cost-cutting . The same website is a starting point for insurances. Many other websites will do comparisons but may or may not be truly independent.
Essentially it's a choice of contract or PAYG (which has several different types and meanings) with one of the big 5 mobile companies or one of the many smaller virtual providers.
Often folk are "forced" to choose a network due to what reception they can get at the location they need. Coverage is based on population density and so low population areas often have poor coverage. Make sure you use a coverage checker for the area you are going to live in.
Other things to consider are flexibility, customer service, multiple mobile deals (if this suits since some give free calls and texts within network which can be good for kids keeping in touch at little or no cost), package deals (like virgin does broadband, TV, landline, mobile as a deal). '3' is stated to be good for data but heard complaints about voice use. All the big mobile companies have their pluses and minuses. Suggestion that uk mobile companies will be forced to allow uk network roaming in future so this should help with coverage issues. I believe they are resisting this meantime despite it being used by foreign visitors to the uk but not permitted except for emergency calls (999/112) to uk residents.
Personally I'm on Giffgaff which is owned by O2, uses their network but run as a separate company. Very flexible tariffs and some other good things but does have limitations as it's a community based operation which means you have limited access to paid staff if problems occur. Pays your money, takes your choice, but I'm happy meantime.