I thought the Special K diet said you can drop a dress size in two weeks? It seems very popular so, if you stick to it, it would probably work.
I think 30 days is long enough to lose a considerable amount of weight if you organise a regime and stick to it, and you don't have to do it in an unhealthy way either. A couple of months ago my family went on a health kick (very short-lived!) and just two weeks of healthy eating made me lose a fair bit of weight, and if you've got more to lose then in the first week or so you should lose a lot more. I wasn't really following any particular diet, I just cut out chocolate, crisps, fizzy drinks (chocolate was tough!) and had loads and loads of vegetables with every meal. That's enough to lose you quite a bit of weight.
I'd say, if you don't go for the Special K diet (though that might be something to give a try for a week or two), simply go the healthy eating way: don't have any chocolate, biscuits, crisps, fizzy drinks (or limit yourself to, say, a couple of squares of good-quality dark chocolate); have three square meals; eat lots of vegetables and drinks lots of water. For your meals, have things like grilled or steamed chicken or fish with things like new potatoes (try not to eat too many carbs, but some are ok), green beans, peas, carrots, mushrooms, tomatoes, asparagus, salad.
If you can, try to do half an hour or so of exercise a day - some cardiac work like running, jogging, cycling, aerobics. It'd also be a good idea to do some toning exercises, like sit-ups, press-ups, leg raises, buttock and thigh clenches etc, because for a lot of people it's toning rather than weight loss they need to work on. Good luck!