How can they know..?

That's simply alarmist nonsense. Britain's weather is governed by series of anticyclones crossing the Atlantic. What makes it so difficult to predict is that we never know until about 2 weeks ahead whether these are going to pass over us, pass to the south or pass to the north. The tropical storms currently wreaking havoc in the Caribbean are part of the anticyclonic system and will eventually reach us. Since the water in the Atlantic is cold, they are likely to lose much of their energy on the way. They might bring strong winds like the current system affecting us, but nothing worse. (I feel a Michael Fish moment coming on)

This is not the same as a forecast, it is merely a probability, based upon comparison with historical weather patterns, and the present situation in the Atlantic.

It is NOT saying something like "we will have a category 2 storm on 23 Sept. and two cat. 3 storms on October 5 and 10". It is a "be prepared" warning that some serious storms are on the way, so that sensible precautions can be taken to mitigate the damage that may be caused if they were completely unexpected.