Technically, you are meant to have only 1 flapjack per day. However, as long as you were having all the extra liquid, I expect you'd be fine. On the Cambridge Diet, people regularly eat more bars than they are meant to and still lose.
However, I am going to be honest with you. If you haven't already bought the flapjacks, I would ask the pharmacist if you can have or buy one to try first. Be aware, they are very hard to eat. Most people (though by no means all) do find them totally inedible - never before have I thrown away £10 worth of food with no qualms lol! Of course, you could get used to it, or dare I say it you might be better off doing the Cambridge Diet where apparently the bars are very nice.
Either way, once you've made your choice, you WILL be able to get through it and get used to it and lose the weight, and probably end up adoring whatever foods you choose by the end.
GOOD LUCK xxx