Really, which mobile phone network you should go for depends on what you want it for. I am pleased with Orange, and find the outdoor coverage excellent, but there is no "best" network. Each will have better coverage in one place or another.
Both GSM 1800 networks offer you the chance to try their system for a fortnight and get a full refund if not satisfied -- less the cost of the calls you made. It would be foolish not to try at least one of them to see it it meet your needs, as the cost savings are bigger than first meets the eye.
If an emergency phone that works in most places, but won't be used much, is your need, a pay-as-you-talk tariff is probably a reasonable bet, but once you've had the convenience of having a mobile phone, you'll probably want to use it, and the call charges on those tariffs can be high! Probably better to get a phone with a year's connection pre-paid.
Comments, suggestions and amendments welcomed!
cellular. v1.53 12 Oct 2000