The Treaty of London is a decision available to 
Netherlands. It should not be confused with Treaty of London (1864).
Historically the Dutch king didn't acknowledge 
Belgium as an independent country until he signed the treaty of London in 1839. Belgium became independent in 1831 by a revolt. Then they were defended by 
France and 
United Kingdom and the Dutch king was forced to sign the treaty.
In the game when the treaty is signed, Netherlands will lose all of its cores in Belgium. Then it will get 10 prestige and improve relations with UK and France by 150, with all other great powers except UK and France by 20 and with Belgium by 25. The treaty can only be signed if the Netherlands have over 100 points of relations with United Kingdom or UK has over 50 influence in the Netherlands. Furthermore they must have researched ideological thought.
Of course Belgium must also exist, but it is not actually required that they own the provinces they start with. So Netherlands can sign the treaty and lose cores after they have reconquered some of them.
The AI will always take the decision after 1839 assuming that Belgium exists, but never before.
Ingame description
While Belgium's de facto independence from The Netherlands had been established by force of arms during the Belgian revolution of 1831, it had not been confirmed de jure by The Netherlands until the signing of the Treaty of London. In 1839 The Netherlands, along with all the Great Powers of Europe, signed the treaty, which formally established Belgium's independence and neutrality. By signing the treaty we will lose our claim to Belgium, but our standing in the eyes of the world will improve.