Good to see the fighting move from Portugal at last to Spain. That said with the losses they have experienced, is it not possible that members of the Spanish army would start privately beginning talks to end the fighting? The cream of the Spanish forces and French just got their butts handed to them over the last few battles. There are advantages to a fighting on home turf. Hopefully you keep this TL going.
I'm going with the OTL Spanish reaction. Some parts will side with France but most of the Spanish forces are going to make a armistice with the Anglo-Portuguese, but a peace treaty will only be signed when the War is over.
Also despite the victories the last battle was brutal for the Anglo-Portuguese Army in the South, too many casualties for no great gains, but they still have the First British division fresh and the FL is also fresh. Also the Portuguese Army of the North and the Spanish Army of Galicia are still intact but the Portuguese is just two divisions strong, and only one for them is made up of peace time battalions, and the Army of Galicia as the numbers but lacks equipment. And the French still have the Beauharnais' army intact and Soult was able to retreat in good order into Spain.
The main problem will be when the Prussian-Russian alliance makes peace with Napoleon, the Anglo-Portuguese forces will have to run to ensure that the roads are under their control before Boney can turn his gaze into Iberia.
The TL is still alive, despite my hiatus, and I plan to post something when my Semester ends, so second week of December, maybe sooner but no promises because I don't have my books with me in Uny.