For the UK GiffGaff is your best bet, its very cheap and you can pay for only 1 month if you like. You can get 250 Minutes, Unlimited texts and 1GB of data for about $15 a month. There are also tariffs with more (or less) if thats what you need.
If it is a .apk file you need to download some sort of file manager from the market (linda or astro for example) and find the file on your phone in there, it should be installable through that.
If you are familiar with ADB you can just do ./adb install (whatever the live wallpaper is called).apk
I'm on Three, where I live the reception is brilliant, 2-4 bars of hspa+ everywhere I go! Ive been with Three with for the last 3 years and their customer service has always been good and helpful.
And the free tethering is just an added bonus!
Currently mine is in a 50p phone sock :p (Which looks like this: Kolay Grey Sock Cover For LG Google Nexus 4: Amazon.co.uk: Electronics ) And I have to say I expected it to be a lot worse than it is for the price, I cant stand to cover my beautiful smooth glass with a horrible bit of plastic so...