Emergency contraception (Plan B) is basically just a megadose of birth control pills, so it's actually more likely to cause nausea, which is the side effect I would think you would most want to avoid on your walima. Not to mention that it has more potential to mess up your cycle, so you might not be able to successfully predict your next ovulation and get pregnant in the next cycle anyway.
Regular birth control pills aren't effective for the first month generally, so if you want to take them you should try to start a couple months in advance. Nadz, in Britain you should be able to get an appointment at a local clinic where you can get a checkup and a few free packs of starter pills, and I'm pretty sure the privacy laws there will protect you from having any information about your visit shared with anyone else if you feel you really can't discuss with anyone. Not sure of your age but if you call the nearest clinic they will tell you if they can give you an appointment without a guardian there.
There have been many studies that show there is little to no link between birth control pills and weight gain. Of course everyone's experience is different, but especially if you're starting the pill only a couple of months before marriage any effect on weight will likely be negligible.
You would do well to plan in advance. Not sure how cooperative your fiance will be with other types of birth control that require his effort, so it's better for you to be in control of this issue. I think it's a much better option than two other strong possibilities that come to mind---either becoming a mom before you're ready, or going for an abortion.