I'm on o2 (have been for years) and just got an S3 as an upgrade - free pebble blue phone, 24 month business contract unlimited calls/texts/data for £30 per month +VAT.
It is the UK based customer services I value and they are very patient with me...........
Oh, and ask for a (free micro-sim adaptor) if like me you wish to use an older phone when going abroad.