Assuming it's as most other cellphones, your cellphone will be what is called, 'SIM Locked' - meaning that it only works on one network and if you put in a SIM-card from an alternative network, it will say 'SIM LOCK', or something similar.
I can't say what network is better... I am only visiting here. My Powertel (US) - now Voicestream - phone is currently using Orange (1800MHz) but depending on signal strength, it's probably easier just to roam on your existing carrier's SIM card and phone - as it will pick up any network available, assuming that your carrier has a roaming agreement with the UK companies.
Also, check that your phone is a 900/1800MHz GSM phone. Assuming it's not a North American GSM phone, you won't have any problems, but most US GSM & iDEN phones will not work in Europe (both systems can use SIM cards).
Just be sure that your phone is not SIM-locked as you could be up a creek without a paddle if it is. (Call your service provider and ask if you are not sure.)
Also, bear in mind that your phone will most likely 'reset' itself to default factory-set options if you replace the SIM-card.
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