With what forces? Where do these come from and what suffers as a result? Its convenient to simply state they would take them, without noting how or with what. Also you aren't factoring in losses, as the Axis will be hitting the invasion force by air and via Uboat too. So while the British could take it, it would come at a cost, the question being what exactly that cost would be.
Losing Gibraltar means Malta falls so the British save resources not having to supply Gibraltar or Malta, but at the cost of being unable to interdict the Axis very well in 1942-43. That also frees up a fairly large amount of Axis resources that were tied up fighting Malta and trying to lock down the Central Mediterranean, so these forces have to be factored in, especially if they show up in the Canary Island campaign.
Plus Axis naval forces could then exit the Mediterranean after Gibraltar can no longer interdict the straights, which means Italian and Spanish naval forces can operate in the Atlantic, especially their submarines. Italy won't need to keep as much of its naval power locked down in the Central Mediterranean either, so it can harass the Eastern Mediterranean or Atlantic at will (and depending on fuel supplies). Britain needs to factor in the resulting pressure on the Vichy French, who now are surrounded by Axis belligerent powers, and the Spanish fleet in the Atlantic.
http://www.kbismarck.com/mgl/cruisers.htm
Plus there now is the threat of German airpower in Spain and Uboats operating from its naval bases. Portugal then has the problem of being next to a belligerent Axis power, which is likely to force it to be even more pro-Axis than it was historically. That could mean Axis spies/units in the Azores.
https://sites.google.com/a/coldie.ne...l/Spanish-Navy
And Spanish merchant raiders, merchant raiders being highly effective in WW2, would now be trolling the Atlantic in higher numbers, putting even more pressure on the RN to escort convoys instead of taking offensive action against the Canaries.