Correct me if Im wrong, but thought Google voice was VOIP (voice over internet protocol) and used with an app? Im not very familiar with it, as GoogleVoice isnt fully available to the UK, we've only just been given access to make calls through GMail. I believe the app should let you make calls over the wifi without any problems. Though if youve come from the US, you should be able to still use it fully like you did back there. Someone who knows more, can correct me if Im wrong.
I would use googletalk at home for calling back to the States as the rates are pretty cheap.
Theres 5 large providers here:
O2,
Vodafone,
3,
Orange &
TMobile (though Orange & TMobile are trying to merge) and all now have the iPhone. To determine coverage, you should be able to enter your post code into each providers site and be able to confirm signal strength. Like home, some areas are better than others at mobile coverage.
When you come over, you may not be able to get a long term monthly contract, without putting a large deposit down, as youll have no credit history. Its okay if you cant, as Pay As You Go plans are pretty competitive with their minutes and texts, with some offers tending to give you a decent amount of data (3 gives you unlimited*) as long as you top up X amount a month. Back home theres a stigma to it, not so much here. Its pretty common thing.
There is no 4G here, the fastest at the moment is 3.5G...
If youre currently using your iPhone on Sprint or Verizon, the phones may not work as the technology the phones use to make a call, is different. You will need a phone thats capable of using the GSM network (think AT&T)
Also a good tip - in the UK on cell/mobile phones we're not charged for incoming calls or texts. Those are free. Youre only charged for what you send or make a call. Also, similar to the US, if you and your other half are on the same provider, youll find minutes/texts all inclusive so youre only charged to connecting with friends on other providers.
Hope this helps.