Great Britain had already broken the Treaty of Amiens and France and Great Britain were at war. Interesting that you left that little bit out of your posting.

The Armee des Cotes de l'Ocean was activated along the Channel coast in six large camps in June 1803. The Treaty of Amiens was broken on or about 18 May 1803. Britain had declared war on France. Do you expect that Napoleon would have done nothing?

The bottom line is that Great Britain chose war in 1803 and that was the reason for the large French army on the Channel coast.