So most colleges send letters to UK students, though some send emails. They'll probably tell you beforehand which way they'll use. International students normally get emails, although the college may ask internationals whether they want an email to be sent (I'm sure I got a general email last year from John's saying that if I am an international student, I can request an email instead of a letter). The letters will be sent out on the 5th January to get to you on the 6th January. The college will probably tell you how long to wait before contacting them directly to ask for information about offers (i.e. if your letter is taking ages to turn up, although this is caused by the mail system rather than the college!), and if this happens they'll probably email you about it. I have talked to the John's admissions department, and they have told me that the size of the letter varies depending on which college you go to, and so will the content of the letter - some colleges send extra information, others don't. Some rejection letters have also been known to contain booklets of things the college think you'll be interested in in the past, so there is really no way of telling by looking at your envelope! You'll just have to open it!*
I know that this wait can be nerve-wracking, and from last year I know that in a way, TSR actually made me more nervous because I could see everyone stressing out on last year's version of this thread. The thing is, there is really nothing you can do once you've finished your interviews, and you'll be much better off trying to enjoy the holidays (and maybe revise for mocks!). I know it's hard, but ultimately everyone I know from my yeargroup has gone on to do things they are really enjoying - a few got in to Oxford, but I also know some who did not, and they are having an amazing time doing other really cool things (one girl has become an apprentice at the BBC studios, for example - how cool is that?). Oxford is cool, but the point is, it's also very small, and the world is much bigger than it. Anyway, you guys should be looking forward to the holidays now, not worrying about offer day yet! Don't wish the holidays away!
Good luck to all those who still have a few interviews left!
*forgot to mention that it's also possible to get an open offer, where you get a place, but your college isn't specified until the summer.