It depends on how full the course is. If they have space, they will usually offer to people who just missed out, especially if you had them as firm choice. If it's full they will only take those who achieved the offer and had them as a firm choice. If it's a reserve choice even meeting the offer doesn't guarentee a place if they are heavily subscribed. I nearly missed out on my firm choice as I was 1 grade down, but thankfully it was an error as they hadn't taken into account the two A levels I'd done that year, not the one I had from the previous year.
The best thing to do is to get in touch as soon as possible after getting your results, as you are unlikely to be the only one who has just missed out, and if they have any spare places after taking all those who have the grades, they will likely offer them on a first come first served basis. They might also offer alternative courses with lower grade requirements (AFAIK you would have to apply through clearing, but they will bascially say they will accept you if you do).
Really though, it's not worth worrying about until you get the results back, and beyond doing the best you can to get the grades there is nothing you can do until then. At this stage almost every course will be oversubscribed, but until the results come out they don't know how many of those will get the grades and how many who have them as a reserve will miss out on their firm.