Germany and France would have been blocked within Europe and become isolated from their colonies. Britain could send as many troops and munitions as it liked, subject only to shipping constraints, to Africa (except for North Africa) to crush the isolated French and German garrisons (once it's defeated the Boers, of course). Ditto for French and German colonies in the Indian and Pacific oceans, and for French colonies in the Caribbean. Canada and Australia would rally to the Crown and Canada would take St. Pierre and Miquelon while Australia and New Zealand (with the help of a few RN cruisers) could begin picking off the many German and French islands in the Pacific. French Indo-China, however, would probably be a bridge too far.
Longtime British ally Portugal might play a supportive role in Africa in return for the Kionga Triangle and a slice of German East Africa (the future Tanzania).
As to Russia, the RN would send the Czar's decrepit navy to DJL (in the unlikely circumstance that the Ottomons ever let a Russian fleet pass through the Bosporus to attack Turkey's then ally Britain).
The anti-British alliance would have no means to invade Britain, although Britain, in turn, would be unable to invade the Continent. All the British have to do is maintain a blockade of the continent and pick off the colonies of its enemies one by one. Assuming Italy stays neutral, there would be no land fighting between the British in Egypt and the French in Algeria, although there would be sea battles. Actually the British could probably get the Italians on their side by offering them Tunisia and French Somaliland, and maybe even a slice of French or German territory on the West African coast. Or give them a couple of German islands in the Pacific, so they can claim to be a Pacific power.
Once the RN defeats the French Mediterranean fleet and crushes an attempt by the German fleet to break out of the Baltic, it will all be over. A blockaded and economically devastated France and Germany sue for peace. Russia--bogged down in Afghanistan and faced by a British Indian Army that is far closer to its supply bases than the Russians could possibly be--has no choice but to follow suit.