tom provided a link to this interesting source 1808, Santiago do Cacém e a Resistência aos Franceses Gentil José Cesário
Which, unfortunately, has this
From the beginning of the invasion, there was an English squadron blocking the mouth of the Tagus, commanded by Admiral Sir Charles Cotton (Fig. 10), 5th Baron of Landwade.
Of course, at the beginning of the invasion, it was Admiral Sir Sidney Smith who was in command and who first established the blockade of the Tagus, . Cook came along to take over, some time later; Smith having returned from escorting the royal bearing ships to blockade Lisbon only because of the presence of the Russian fleet in that harbor.
As far as I can determine, especially at the beginning, the only reason the blockade was ever established was due to concerns over the Russian fleet escaping. After all, there was nothing else there. The Portuguese fleet had left for Rio. Other than the Russians, there was nothing there to blockade. Yours and every other source I've seen makes this same basic mistake. Without this information, it's is misleading the readers to think that the British blockade was some sort of economic blockade directed specifically against Portugal.