The reason why the English had interests in France but not vice vera was that they acquired feudal demesnes. Any enclaves come from these or the results of wars.
There is no reason though why the French king could not have acquired one. After all until 1237 the King of Scotland was the Earl of Huntingdon. We would then have the interesting situation of two medieval monarchs swearing fealty each other for land holding in other's kingdom.
The advantage of the French king not having demesnes or enclaves in England was that on the feudal scale he could claim superiority over his English counterpart because he was the other's overlord. In addition, the costs of maintianing such holdings often outweighed their economic returns.
The points made on the French fleet are important. They had three fronts to fight on east against the HRE, south against the Cathars and Spanish and north against the English. In contrast the English only had two, north against the Scots and south against the French. In addition much trade was with the continent. It is that not surprising that although their kingdom was poorer the English had more ships. A French enclave at Cowes or Hastings would thus be less sustainable per se than Calais.