Help with the Nexus 4 in the UK

Hey guys, I need some help.

I am moving to the UK January 1st 2013. I will spend a year in Birmingham as an exchange student. I currently live in Brazil, where Google does not sell devices in the Play Store and neither does LG sell the Nexus 4 on the high street.

I am really inclined to buy the Nexus 4 for all the obvious reasons, but mainly to get rid of my year old Motorola XT531. But I have oh so many questions, so could you guys help?

What are the payment methods accepted by the Play Store for devices? It might be a long time until I get a credit or a debit card issued in the UK, so I am looking for some alternatives such as a bank payment slip (is this correct? I translated it from Google). Or can I pay it with my AmEx GlobalCard (debit card) that is issued in Brazil but works in Pounds? Is it enough to have a delivery address in the UK or should the card also be issued in the UK?

Second question: I don't know what carrier to use. First of all just let me say O2 really does not want money. Can you believe that their website won't let me check their plans and offerings just because I am not browsing from the UK? I don't know if that works out for the States as well, but man, I don't think it is a very logical thing to do. I was looking ahead at this and it seems the best carrier is Three. Is this true?

I want the Ultimate Internet SIM 200 1 Month pay monthly plan. Its 12,90 GBP. Is it expensive? I really want all you can eat data, cheap and fast. Here in Brazil, carriers charge absurd prices for these services, so the Three plan is cheaper than anything over here. But is there another better alternative? Oh yeah, I am going to the UK with a scholarship, so I am not the usual rich exchange student.

On an unrelated note, I also have a grant to purchase a notbook computer. I am thinking Macbook Air 13'' with the student discount. It is the best option right? Can I buy it on the Apple Retail Store with the discount or only on the Apple Online Store? Just showing my student ID is enough to award me the discount?