Tesco Mobile has released iPhone 4S Price and Tariff Plan in the UK. Two contract lengths are available Current time which allows you to select either 12 or 24 months contracts . On the 12 month contracts you can opt to pay £25/month or £35/month and for that you get either 250 or 750 minutes along with 5000 text messages and 1GB of data.
On the 24 month contracts you can get the 16GB model free if you pay £45/month and for that you get 3000 minutes along with 5000 text messages. All plans get the same 1GB of data.
Tesco Mobile network uses the O2 network, so if you know O2 gets a good signal where you live then Tesco Mobile devices will also get a good signal.
You can register your interest over here.