It most definitely regarded it as dangerous even when the port was almost unusable. But Britain also considered Russian presence in the Central Asia as a danger to their possessions in India (more than 1000 miles away). The issue is not existence or absence of a fear but its foundation in a reality. If the fear is based on something real (for example, clear aggression) it is a good excuse for an action. If it is just fear for fear's sake, it is paranoia.
If so it was a remarkably long-lived paranoia, apparently shared by virtually every British government and people from Louis XIV's day through WW1 - a span of over two centuries!
Personally, I find it easier to assume that those concerned were correct in their assumptions, whatever someone or other with a couple of centuries hindsight may choose to believe. But in any case the point is moot. If that is what just about everyone
did believe at the time, then that was what would determine their actions. Postmortems to the contrary are irrelevant.
Of course, even if they were wrong about Antwerp, it doesn't change the fact that France - whether ruled by Republicans, Royalists, Bonapartists or Seventh Day Adventists - was still Britain's biggest imperial rival, so we'd still have an interest in preventing it from making major gains of territory and power, even if the port of Antwerp had never existed.
BTW, wasn't it
Napoleon who referred to Antwerp as a pistol pointed at the heart of England? If this notion was merely British paranoia, how did a Corsican general, who never set foot in Britain in his life, come to share it?