Complex sentence: Although outgunned by "Graf Spee" and therefore at a tactical disadvantage, the British did have the upper hand strategically since any raider returning to Germany would have to run the blockade of the North Sea and might reasonably be expected to encounter the Home Fleet.
Split into simpler sentences: Although outgunned by "Graf Spee" and therefore at a tactical disadvantage, the British did have the upper hand strategically. Since any raider returning to Germany would have to run the blockade of the North Sea and it might reasonably be expected that an engagement would therefore take place with the Home Fleet.