There really is a huge amount of risk and not much chance of gain for Franco.
Yabbut WI Franco is not the Caudillo? He was the third in line after Sanjurjo and Mola, both of whom died in completely avoidable airplane crashes. (Sanjurjo's being especially stupid and his own fault.) Was either of them known to be interested in Spanish annexation of Portugal?
Also, if Spain moves in fall 1940, I don't think Britain can do much of anything. Maybe after they
know SEELOWE has been cancelled; but their means are so slender and other needs so great that sparing anything for Portugal seems right out until 1941. Furthermore, Britain aiding Portugal with troops probably means Spain explicltly joining the Axis. (As opposed to independently exploiting the distraction and weakness of Britain and France.)
Britain has a lot to lose if that happens, and won't want to risk it.
It could be analogous to the decision in 1939-1940 to overlook the Soviet attack on Poland. "One war at a time" was the mantra.
There could be interesting consequences though. Salazar would evacuate to the Azores, and retain control of the colonies. He could offer Britain use of the Azores for ASW basing, which would be very helpful, and would not force the issue between Britain and Spain. Madeira, Cape Verde, and Guinea would also be useful.
Supposing that the U.S. enters the war about as OTL - the Azores and Madeiras would be important cover/bases for an Allied invasion of French North Africa. Spain could still remain nominally neutral, but would see "the handwriting on the wall": Portugal's alliance with the Allies points to an eventual Allied invasion of Spain for restoration of Portugal
and the Republic.
Spain would then have the choice of joining the Axis and intervening in North Africa (jointly with Germany and Italy) or backing off and withdrawing from Portugal, thereby removing the
casus belli. The latter would probably be unthinkable to a Caudillo foolish enough to move on Portugal in the first place, so the former seems more likely.
The North African campaign then gets stretched out into later 1943, and will be followed by an Iberian campaign.
Or (wild-card possibility) the folly of joining the Axis at this stage of the war is obvious to others in the Spanish junta, who stage a coup. Franco comes to power with a mandate to get on the right side of the Allies, whatever it takes. That begins with withdrawing from Portugal, and continues with
joining the Allies. (Is it crazy? Italy and Romania both did it.) Franco IMHO was ruthless enough to do it. Churchill if not FDR might be cold enough to make the deal (Spain joins the Allies, the Allies leave the junta in power). As with OTL Italy, all the bad acts can be blamed on the late (?) ex-Caudillo.
If it happens, Spain becomes the beachhead for the Allies on the continent, with multiple major ports to use for supply. The transport net to the front in France was battered in the Civil War, but it's been patched up in the last four years, And it
hasn't been worked over by Bomber Command and the Mighty 8th. The liberation of western Europe could start at the Pyrenees in mid-1943 instead of the Italian campaign.