If war breaks out in 1900 between Britain and Germany the fighting would go like this:
Britain would take the German colonies. These are worthless trophies that won't hurt Germany one bit. The Germans will start attacking British shipping using armed merchant ships, cruisers and the like
The French and Russians will seek to keep the war going as long as possible and grab what they can as their two main rivals fight it out. They'll look for easy gains as the British and Germans seek them as allies. The Japanese would be prime targets of the Russians. Expect an ultimatium reducing them to Russian protectorate. Without British aid and France backing them, the Japanese will bend or be quickly defeated
The French will seek colonial gains- Morocco for sure. The Portuguese colonies would also be possible. More risky but possible would be the Belgian Congo and Indonesia
Without the Haber-Bosch Process (industrialized 1913) won't the blockade defeat Germany rather quickly when the run out of ammunition for their weapons?
Unless the British somehow get the French and Russians to join in (rather unlikely) there won't be any blockade of Germany. The French and Russians despise the British as much as the Germans at this point and would have no reason to back Britain. Let the two rivals fight it out
As to Haber-Bosch- in WW I this was crucial as German agriculture depended on artificial fertilizers. The cutting off of nitrates gave the Germans a food or bombs dilemma. This was made worse by having their horses and men sent to the front. Assuming Franco-Russian neutrality, there is no problem. Russian grain would be used as it was before the war.
What little nitrates they would need could come from either natural sources (animal dung), imports through France and Russia or as a byproduct of the coking process