But the power the UK was afraid of immediately before WW1 was France.

This isn't correct - British planning immediately before WW1 always saw Germany as the bigger threat. The threat from France had been most significant in the mid-19th Century and earlier. However, as the 20th Century began, Germany became the most significant threat to the UK, mainly due expansion of its navy, its ability to disrupt the balance of power on the continent and its desire for a colonial empire.

So, why were the Belgian ports more significant than other ports on the coast? The answer lies primarily in the location of the naval bases on the south coast of England. The two main bases of the Royal Navy in the 19th Century were at Plymouth and Portsmouth. These are towards the western end of the English Channel, and are excellently positioned to deal with any threat from France. But towards the eastern end of the Channel, where it narrows, there were no naval bases. The Belgian ports were the nearest major ports to the Straits of Dover. Ships operating from them could threaten coastal shipping carrying goods from the major ports on the west coast to London, or they could block the Dover Straits - preventing the RN from moving eastwards from its bases, making an invasion much easier. None of the other ports on the Channel coast really allow such activity; the French ports are both too far away for a rapid strike, and neutralised by Portsmouth and Plymouth, while the Dutch ports are poorly positioned, and can be threatened from bases in Scotland.