Probably a lot later than you might want but in the 1870's-1880's the two faced off over central South Africa. Lisbon hoped to connect its Angola and Mozambique colonies overland, while Britian's dreams of Cape-to-Cairo made this a little tricky.
In OTL Portugal saw sense and backed down from Britannia but perhaps localised fighting sparks something? Strikes me as an interesting parrallel to the Spanish-American War.