I've been to midnight launches at asda a few times and once at gamestation. They'll have more than enough copies as they need alot of extra copies to stock the shelves the next day as well.
For a game like CoD you'll need to get there a good hour and a half/2 hours early, it's normally the busiest launch in my experience. The queue is normally up to the door when midnight comes.
If you have a supermarket like asda locally go there instead of gamestation/game. It shouldn't be quite as busy and might be a bit cheaper too.