The latest I have heard is o2 do not do any Blackberry phones, and Vodafone are doing the Passport for business users only. You would have to get the phone from carphone warehouse or someone similar, on a contract with o2 or vodafone. I am stuck with an o2 business phone so will have to look at another phone NOT HAPPY.
This depends on signal strength in your area, I'm on o2 because it is a work phone. The rest of my family use Vodafone, the later have better option when going to Europe. For 3 pounds a day you can take your UK texts Mins and data with you, on o2 it was £1.66+Vat for 50mb of data, 42p for a 1 hour call and 9p a text. I would go Vodafone if I was taking a personal phone. This is just my opinion though.