The royal Navy consists of 27 ships, most of which are small and incapable of being part of a blockage. We couldn't blockade Calais and Dunkirk, let alone all the others.

Also, Gibraltar is still armed to the teeth and heavily dug in. Occupying it would probably require a significant mobilization of the Spanish army and the casualties would be colossal. A Pyrrhic victory at best

keep to the Fen Causeway